tuplesort.c

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/*-------------------------------------------------------------------------
 *
 * tuplesort.c
 *    Generalized tuple sorting routines.
 *
 * This module handles sorting of heap tuples, index tuples, or single
 * Datums (and could easily support other kinds of sortable objects,
 * if necessary).  It works efficiently for both small and large amounts
 * of data.  Small amounts are sorted in-memory using qsort().  Large
 * amounts are sorted using temporary files and a standard external sort
 * algorithm.
 *
 * See Knuth, volume 3, for more than you want to know about the external
 * sorting algorithm.  We divide the input into sorted runs using replacement
 * selection, in the form of a priority tree implemented as a heap
 * (essentially his Algorithm 5.2.3H), then merge the runs using polyphase
 * merge, Knuth's Algorithm 5.4.2D.  The logical "tapes" used by Algorithm D
 * are implemented by logtape.c, which avoids space wastage by recycling
 * disk space as soon as each block is read from its "tape".
 *
 * We do not form the initial runs using Knuth's recommended replacement
 * selection data structure (Algorithm 5.4.1R), because it uses a fixed
 * number of records in memory at all times.  Since we are dealing with
 * tuples that may vary considerably in size, we want to be able to vary
 * the number of records kept in memory to ensure full utilization of the
 * allowed sort memory space.  So, we keep the tuples in a variable-size
 * heap, with the next record to go out at the top of the heap.  Like
 * Algorithm 5.4.1R, each record is stored with the run number that it
 * must go into, and we use (run number, key) as the ordering key for the
 * heap.  When the run number at the top of the heap changes, we know that
 * no more records of the prior run are left in the heap.
 *
 * The approximate amount of memory allowed for any one sort operation
 * is specified in kilobytes by the caller (most pass work_mem).  Initially,
 * we absorb tuples and simply store them in an unsorted array as long as
 * we haven't exceeded workMem.  If we reach the end of the input without
 * exceeding workMem, we sort the array using qsort() and subsequently return
 * tuples just by scanning the tuple array sequentially.  If we do exceed
 * workMem, we construct a heap using Algorithm H and begin to emit tuples
 * into sorted runs in temporary tapes, emitting just enough tuples at each
 * step to get back within the workMem limit.  Whenever the run number at
 * the top of the heap changes, we begin a new run with a new output tape
 * (selected per Algorithm D).  After the end of the input is reached,
 * we dump out remaining tuples in memory into a final run (or two),
 * then merge the runs using Algorithm D.
 *
 * When merging runs, we use a heap containing just the frontmost tuple from
 * each source run; we repeatedly output the smallest tuple and insert the
 * next tuple from its source tape (if any).  When the heap empties, the merge
 * is complete.  The basic merge algorithm thus needs very little memory ---
 * only M tuples for an M-way merge, and M is constrained to a small number.
 * However, we can still make good use of our full workMem allocation by
 * pre-reading additional tuples from each source tape.  Without prereading,
 * our access pattern to the temporary file would be very erratic; on average
 * we'd read one block from each of M source tapes during the same time that
 * we're writing M blocks to the output tape, so there is no sequentiality of
 * access at all, defeating the read-ahead methods used by most Unix kernels.
 * Worse, the output tape gets written into a very random sequence of blocks
 * of the temp file, ensuring that things will be even worse when it comes
 * time to read that tape.  A straightforward merge pass thus ends up doing a
 * lot of waiting for disk seeks.  We can improve matters by prereading from
 * each source tape sequentially, loading about workMem/M bytes from each tape
 * in turn.  Then we run the merge algorithm, writing but not reading until
 * one of the preloaded tuple series runs out.  Then we switch back to preread
 * mode, fill memory again, and repeat.  This approach helps to localize both
 * read and write accesses.
 *
 * When the caller requests random access to the sort result, we form
 * the final sorted run on a logical tape which is then "frozen", so
 * that we can access it randomly.  When the caller does not need random
 * access, we return from tuplesort_performsort() as soon as we are down
 * to one run per logical tape.  The final merge is then performed
 * on-the-fly as the caller repeatedly calls tuplesort_getXXX; this
 * saves one cycle of writing all the data out to disk and reading it in.
 *
 * Before Postgres 8.2, we always used a seven-tape polyphase merge, on the
 * grounds that 7 is the "sweet spot" on the tapes-to-passes curve according
 * to Knuth's figure 70 (section 5.4.2).  However, Knuth is assuming that
 * tape drives are expensive beasts, and in particular that there will always
 * be many more runs than tape drives.  In our implementation a "tape drive"
 * doesn't cost much more than a few Kb of memory buffers, so we can afford
 * to have lots of them.  In particular, if we can have as many tape drives
 * as sorted runs, we can eliminate any repeated I/O at all.  In the current
 * code we determine the number of tapes M on the basis of workMem: we want
 * workMem/M to be large enough that we read a fair amount of data each time
 * we preread from a tape, so as to maintain the locality of access described
 * above.  Nonetheless, with large workMem we can have many tapes.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/sort/tuplesort.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>

#include "access/htup_details.h"
#include "access/nbtree.h"
#include "catalog/index.h"
#include "commands/tablespace.h"
#include "executor/executor.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "utils/datum.h"
#include "utils/logtape.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/pg_rusage.h"
#include "utils/rel.h"
#include "utils/sortsupport.h"
#include "utils/tuplesort.h"


/* sort-type codes for sort__start probes */
#define HEAP_SORT       0
#define INDEX_SORT      1
#define DATUM_SORT      2
#define CLUSTER_SORT    3

/* GUC variables */
#ifdef TRACE_SORT
bool        trace_sort = false;
#endif

#ifdef DEBUG_BOUNDED_SORT
bool        optimize_bounded_sort = true;
#endif


/*
 * The objects we actually sort are SortTuple structs.  These contain
 * a pointer to the tuple proper (might be a MinimalTuple or IndexTuple),
 * which is a separate palloc chunk --- we assume it is just one chunk and
 * can be freed by a simple pfree().  SortTuples also contain the tuple's
 * first key column in Datum/nullflag format, and an index integer.
 *
 * Storing the first key column lets us save heap_getattr or index_getattr
 * calls during tuple comparisons.  We could extract and save all the key
 * columns not just the first, but this would increase code complexity and
 * overhead, and wouldn't actually save any comparison cycles in the common
 * case where the first key determines the comparison result.  Note that
 * for a pass-by-reference datatype, datum1 points into the "tuple" storage.
 *
 * When sorting single Datums, the data value is represented directly by
 * datum1/isnull1.  If the datatype is pass-by-reference and isnull1 is false,
 * then datum1 points to a separately palloc'd data value that is also pointed
 * to by the "tuple" pointer; otherwise "tuple" is NULL.
 *
 * While building initial runs, tupindex holds the tuple's run number.  During
 * merge passes, we re-use it to hold the input tape number that each tuple in
 * the heap was read from, or to hold the index of the next tuple pre-read
 * from the same tape in the case of pre-read entries.  tupindex goes unused
 * if the sort occurs entirely in memory.
 */
typedef struct
{
    void       *tuple;          /* the tuple proper */
    Datum       datum1;         /* value of first key column */
    bool        isnull1;        /* is first key column NULL? */
    int         tupindex;       /* see notes above */
} SortTuple;


/*
 * Possible states of a Tuplesort object.  These denote the states that
 * persist between calls of Tuplesort routines.
 */
typedef enum
{
    TSS_INITIAL,                /* Loading tuples; still within memory limit */
    TSS_BOUNDED,                /* Loading tuples into bounded-size heap */
    TSS_BUILDRUNS,              /* Loading tuples; writing to tape */
    TSS_SORTEDINMEM,            /* Sort completed entirely in memory */
    TSS_SORTEDONTAPE,           /* Sort completed, final run is on tape */
    TSS_FINALMERGE              /* Performing final merge on-the-fly */
} TupSortStatus;

/*
 * Parameters for calculation of number of tapes to use --- see inittapes()
 * and tuplesort_merge_order().
 *
 * In this calculation we assume that each tape will cost us about 3 blocks
 * worth of buffer space (which is an underestimate for very large data
 * volumes, but it's probably close enough --- see logtape.c).
 *
 * MERGE_BUFFER_SIZE is how much data we'd like to read from each input
 * tape during a preread cycle (see discussion at top of file).
 */
#define MINORDER        6       /* minimum merge order */
#define TAPE_BUFFER_OVERHEAD        (BLCKSZ * 3)
#define MERGE_BUFFER_SIZE           (BLCKSZ * 32)

typedef int (*SortTupleComparator) (const SortTuple *a, const SortTuple *b,
                                                Tuplesortstate *state);

/*
 * Private state of a Tuplesort operation.
 */
struct Tuplesortstate
{
    TupSortStatus status;       /* enumerated value as shown above */
    int         nKeys;          /* number of columns in sort key */
    bool        randomAccess;   /* did caller request random access? */
    bool        bounded;        /* did caller specify a maximum number of
                                 * tuples to return? */
    bool        boundUsed;      /* true if we made use of a bounded heap */
    int         bound;          /* if bounded, the maximum number of tuples */
    long        availMem;       /* remaining memory available, in bytes */
    long        allowedMem;     /* total memory allowed, in bytes */
    int         maxTapes;       /* number of tapes (Knuth's T) */
    int         tapeRange;      /* maxTapes-1 (Knuth's P) */
    MemoryContext sortcontext;  /* memory context holding all sort data */
    LogicalTapeSet *tapeset;    /* logtape.c object for tapes in a temp file */

    /*
     * These function pointers decouple the routines that must know what kind
     * of tuple we are sorting from the routines that don't need to know it.
     * They are set up by the tuplesort_begin_xxx routines.
     *
     * Function to compare two tuples; result is per qsort() convention, ie:
     * <0, 0, >0 according as a<b, a=b, a>b.  The API must match
     * qsort_arg_comparator.
     */
    SortTupleComparator comparetup;

    /*
     * Function to copy a supplied input tuple into palloc'd space and set up
     * its SortTuple representation (ie, set tuple/datum1/isnull1).  Also,
     * state->availMem must be decreased by the amount of space used for the
     * tuple copy (note the SortTuple struct itself is not counted).
     */
    void        (*copytup) (Tuplesortstate *state, SortTuple *stup, void *tup);

    /*
     * Function to write a stored tuple onto tape.  The representation of the
     * tuple on tape need not be the same as it is in memory; requirements on
     * the tape representation are given below.  After writing the tuple,
     * pfree() the out-of-line data (not the SortTuple struct!), and increase
     * state->availMem by the amount of memory space thereby released.
     */
    void        (*writetup) (Tuplesortstate *state, int tapenum,
                                         SortTuple *stup);

    /*
     * Function to read a stored tuple from tape back into memory. 'len' is
     * the already-read length of the stored tuple.  Create a palloc'd copy,
     * initialize tuple/datum1/isnull1 in the target SortTuple struct, and
     * decrease state->availMem by the amount of memory space consumed.
     */
    void        (*readtup) (Tuplesortstate *state, SortTuple *stup,
                                        int tapenum, unsigned int len);

    /*
     * Function to reverse the sort direction from its current state. (We
     * could dispense with this if we wanted to enforce that all variants
     * represent the sort key information alike.)
     */
    void        (*reversedirection) (Tuplesortstate *state);

    /*
     * This array holds the tuples now in sort memory.  If we are in state
     * INITIAL, the tuples are in no particular order; if we are in state
     * SORTEDINMEM, the tuples are in final sorted order; in states BUILDRUNS
     * and FINALMERGE, the tuples are organized in "heap" order per Algorithm
     * H.  (Note that memtupcount only counts the tuples that are part of the
     * heap --- during merge passes, memtuples[] entries beyond tapeRange are
     * never in the heap and are used to hold pre-read tuples.)  In state
     * SORTEDONTAPE, the array is not used.
     */
    SortTuple  *memtuples;      /* array of SortTuple structs */
    int         memtupcount;    /* number of tuples currently present */
    int         memtupsize;     /* allocated length of memtuples array */
    bool        growmemtuples;  /* memtuples' growth still underway? */

    /*
     * While building initial runs, this is the current output run number
     * (starting at 0).  Afterwards, it is the number of initial runs we made.
     */
    int         currentRun;

    /*
     * Unless otherwise noted, all pointer variables below are pointers to
     * arrays of length maxTapes, holding per-tape data.
     */

    /*
     * These variables are only used during merge passes.  mergeactive[i] is
     * true if we are reading an input run from (actual) tape number i and
     * have not yet exhausted that run.  mergenext[i] is the memtuples index
     * of the next pre-read tuple (next to be loaded into the heap) for tape
     * i, or 0 if we are out of pre-read tuples.  mergelast[i] similarly
     * points to the last pre-read tuple from each tape.  mergeavailslots[i]
     * is the number of unused memtuples[] slots reserved for tape i, and
     * mergeavailmem[i] is the amount of unused space allocated for tape i.
     * mergefreelist and mergefirstfree keep track of unused locations in the
     * memtuples[] array.  The memtuples[].tupindex fields link together
     * pre-read tuples for each tape as well as recycled locations in
     * mergefreelist. It is OK to use 0 as a null link in these lists, because
     * memtuples[0] is part of the merge heap and is never a pre-read tuple.
     */
    bool       *mergeactive;    /* active input run source? */
    int        *mergenext;      /* first preread tuple for each source */
    int        *mergelast;      /* last preread tuple for each source */
    int        *mergeavailslots;    /* slots left for prereading each tape */
    long       *mergeavailmem;  /* availMem for prereading each tape */
    int         mergefreelist;  /* head of freelist of recycled slots */
    int         mergefirstfree; /* first slot never used in this merge */

    /*
     * Variables for Algorithm D.  Note that destTape is a "logical" tape
     * number, ie, an index into the tp_xxx[] arrays.  Be careful to keep
     * "logical" and "actual" tape numbers straight!
     */
    int         Level;          /* Knuth's l */
    int         destTape;       /* current output tape (Knuth's j, less 1) */
    int        *tp_fib;         /* Target Fibonacci run counts (A[]) */
    int        *tp_runs;        /* # of real runs on each tape */
    int        *tp_dummy;       /* # of dummy runs for each tape (D[]) */
    int        *tp_tapenum;     /* Actual tape numbers (TAPE[]) */
    int         activeTapes;    /* # of active input tapes in merge pass */

    /*
     * These variables are used after completion of sorting to keep track of
     * the next tuple to return.  (In the tape case, the tape's current read
     * position is also critical state.)
     */
    int         result_tape;    /* actual tape number of finished output */
    int         current;        /* array index (only used if SORTEDINMEM) */
    bool        eof_reached;    /* reached EOF (needed for cursors) */

    /* markpos_xxx holds marked position for mark and restore */
    long        markpos_block;  /* tape block# (only used if SORTEDONTAPE) */
    int         markpos_offset; /* saved "current", or offset in tape block */
    bool        markpos_eof;    /* saved "eof_reached" */

    /*
     * These variables are specific to the MinimalTuple case; they are set by
     * tuplesort_begin_heap and used only by the MinimalTuple routines.
     */
    TupleDesc   tupDesc;
    SortSupport sortKeys;       /* array of length nKeys */

    /*
     * This variable is shared by the single-key MinimalTuple case and the
     * Datum case (which both use qsort_ssup()).  Otherwise it's NULL.
     */
    SortSupport onlyKey;

    /*
     * These variables are specific to the CLUSTER case; they are set by
     * tuplesort_begin_cluster.  Note CLUSTER also uses tupDesc and
     * indexScanKey.
     */
    IndexInfo  *indexInfo;      /* info about index being used for reference */
    EState     *estate;         /* for evaluating index expressions */

    /*
     * These variables are specific to the IndexTuple case; they are set by
     * tuplesort_begin_index_xxx and used only by the IndexTuple routines.
     */
    Relation    heapRel;        /* table the index is being built on */
    Relation    indexRel;       /* index being built */

    /* These are specific to the index_btree subcase: */
    ScanKey     indexScanKey;
    bool        enforceUnique;  /* complain if we find duplicate tuples */

    /* These are specific to the index_hash subcase: */
    uint32      hash_mask;      /* mask for sortable part of hash code */

    /*
     * These variables are specific to the Datum case; they are set by
     * tuplesort_begin_datum and used only by the DatumTuple routines.
     */
    Oid         datumType;
    /* we need typelen and byval in order to know how to copy the Datums. */
    int         datumTypeLen;
    bool        datumTypeByVal;

    /*
     * Resource snapshot for time of sort start.
     */
#ifdef TRACE_SORT
    PGRUsage    ru_start;
#endif
};

#define COMPARETUP(state,a,b)   ((*(state)->comparetup) (a, b, state))
#define COPYTUP(state,stup,tup) ((*(state)->copytup) (state, stup, tup))
#define WRITETUP(state,tape,stup)   ((*(state)->writetup) (state, tape, stup))
#define READTUP(state,stup,tape,len) ((*(state)->readtup) (state, stup, tape, len))
#define REVERSEDIRECTION(state) ((*(state)->reversedirection) (state))
#define LACKMEM(state)      ((state)->availMem < 0)
#define USEMEM(state,amt)   ((state)->availMem -= (amt))
#define FREEMEM(state,amt)  ((state)->availMem += (amt))

/*
 * NOTES about on-tape representation of tuples:
 *
 * We require the first "unsigned int" of a stored tuple to be the total size
 * on-tape of the tuple, including itself (so it is never zero; an all-zero
 * unsigned int is used to delimit runs).  The remainder of the stored tuple
 * may or may not match the in-memory representation of the tuple ---
 * any conversion needed is the job of the writetup and readtup routines.
 *
 * If state->randomAccess is true, then the stored representation of the
 * tuple must be followed by another "unsigned int" that is a copy of the
 * length --- so the total tape space used is actually sizeof(unsigned int)
 * more than the stored length value.  This allows read-backwards.  When
 * randomAccess is not true, the write/read routines may omit the extra
 * length word.
 *
 * writetup is expected to write both length words as well as the tuple
 * data.  When readtup is called, the tape is positioned just after the
 * front length word; readtup must read the tuple data and advance past
 * the back length word (if present).
 *
 * The write/read routines can make use of the tuple description data
 * stored in the Tuplesortstate record, if needed.  They are also expected
 * to adjust state->availMem by the amount of memory space (not tape space!)
 * released or consumed.  There is no error return from either writetup
 * or readtup; they should ereport() on failure.
 *
 *
 * NOTES about memory consumption calculations:
 *
 * We count space allocated for tuples against the workMem limit, plus
 * the space used by the variable-size memtuples array.  Fixed-size space
 * is not counted; it's small enough to not be interesting.
 *
 * Note that we count actual space used (as shown by GetMemoryChunkSpace)
 * rather than the originally-requested size.  This is important since
 * palloc can add substantial overhead.  It's not a complete answer since
 * we won't count any wasted space in palloc allocation blocks, but it's
 * a lot better than what we were doing before 7.3.
 */

/* When using this macro, beware of double evaluation of len */
#define LogicalTapeReadExact(tapeset, tapenum, ptr, len) \
    do { \
        if (LogicalTapeRead(tapeset, tapenum, ptr, len) != (size_t) (len)) \
            elog(ERROR, "unexpected end of data"); \
    } while(0)


static Tuplesortstate *tuplesort_begin_common(int workMem, bool randomAccess);
static void puttuple_common(Tuplesortstate *state, SortTuple *tuple);
static void inittapes(Tuplesortstate *state);
static void selectnewtape(Tuplesortstate *state);
static void mergeruns(Tuplesortstate *state);
static void mergeonerun(Tuplesortstate *state);
static void beginmerge(Tuplesortstate *state);
static void mergepreread(Tuplesortstate *state);
static void mergeprereadone(Tuplesortstate *state, int srcTape);
static void dumptuples(Tuplesortstate *state, bool alltuples);
static void make_bounded_heap(Tuplesortstate *state);
static void sort_bounded_heap(Tuplesortstate *state);
static void tuplesort_heap_insert(Tuplesortstate *state, SortTuple *tuple,
                      int tupleindex, bool checkIndex);
static void tuplesort_heap_siftup(Tuplesortstate *state, bool checkIndex);
static unsigned int getlen(Tuplesortstate *state, int tapenum, bool eofOK);
static void markrunend(Tuplesortstate *state, int tapenum);
static int comparetup_heap(const SortTuple *a, const SortTuple *b,
                Tuplesortstate *state);
static void copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_heap(Tuplesortstate *state, int tapenum,
              SortTuple *stup);
static void readtup_heap(Tuplesortstate *state, SortTuple *stup,
             int tapenum, unsigned int len);
static void reversedirection_heap(Tuplesortstate *state);
static int comparetup_cluster(const SortTuple *a, const SortTuple *b,
                   Tuplesortstate *state);
static void copytup_cluster(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_cluster(Tuplesortstate *state, int tapenum,
                 SortTuple *stup);
static void readtup_cluster(Tuplesortstate *state, SortTuple *stup,
                int tapenum, unsigned int len);
static int comparetup_index_btree(const SortTuple *a, const SortTuple *b,
                       Tuplesortstate *state);
static int comparetup_index_hash(const SortTuple *a, const SortTuple *b,
                      Tuplesortstate *state);
static void copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_index(Tuplesortstate *state, int tapenum,
               SortTuple *stup);
static void readtup_index(Tuplesortstate *state, SortTuple *stup,
              int tapenum, unsigned int len);
static void reversedirection_index_btree(Tuplesortstate *state);
static void reversedirection_index_hash(Tuplesortstate *state);
static int comparetup_datum(const SortTuple *a, const SortTuple *b,
                 Tuplesortstate *state);
static void copytup_datum(Tuplesortstate *state, SortTuple *stup, void *tup);
static void writetup_datum(Tuplesortstate *state, int tapenum,
               SortTuple *stup);
static void readtup_datum(Tuplesortstate *state, SortTuple *stup,
              int tapenum, unsigned int len);
static void reversedirection_datum(Tuplesortstate *state);
static void free_sort_tuple(Tuplesortstate *state, SortTuple *stup);

/*
 * Special versions of qsort just for SortTuple objects.  qsort_tuple() sorts
 * any variant of SortTuples, using the appropriate comparetup function.
 * qsort_ssup() is specialized for the case where the comparetup function
 * reduces to ApplySortComparator(), that is single-key MinimalTuple sorts
 * and Datum sorts.
 */
#include "qsort_tuple.c"


/*
 *      tuplesort_begin_xxx
 *
 * Initialize for a tuple sort operation.
 *
 * After calling tuplesort_begin, the caller should call tuplesort_putXXX
 * zero or more times, then call tuplesort_performsort when all the tuples
 * have been supplied.  After performsort, retrieve the tuples in sorted
 * order by calling tuplesort_getXXX until it returns false/NULL.  (If random
 * access was requested, rescan, markpos, and restorepos can also be called.)
 * Call tuplesort_end to terminate the operation and release memory/disk space.
 *
 * Each variant of tuplesort_begin has a workMem parameter specifying the
 * maximum number of kilobytes of RAM to use before spilling data to disk.
 * (The normal value of this parameter is work_mem, but some callers use
 * other values.)  Each variant also has a randomAccess parameter specifying
 * whether the caller needs non-sequential access to the sort result.
 */

static Tuplesortstate *
tuplesort_begin_common(int workMem, bool randomAccess)
{
    Tuplesortstate *state;
    MemoryContext sortcontext;
    MemoryContext oldcontext;

    /*
     * Create a working memory context for this sort operation. All data
     * needed by the sort will live inside this context.
     */
    sortcontext = AllocSetContextCreate(CurrentMemoryContext,
                                        "TupleSort",
                                        ALLOCSET_DEFAULT_MINSIZE,
                                        ALLOCSET_DEFAULT_INITSIZE,
                                        ALLOCSET_DEFAULT_MAXSIZE);

    /*
     * Make the Tuplesortstate within the per-sort context.  This way, we
     * don't need a separate pfree() operation for it at shutdown.
     */
    oldcontext = MemoryContextSwitchTo(sortcontext);

    state = (Tuplesortstate *) palloc0(sizeof(Tuplesortstate));

#ifdef TRACE_SORT
    if (trace_sort)
        pg_rusage_init(&state->ru_start);
#endif

    state->status = TSS_INITIAL;
    state->randomAccess = randomAccess;
    state->bounded = false;
    state->boundUsed = false;
    state->allowedMem = workMem * 1024L;
    state->availMem = state->allowedMem;
    state->sortcontext = sortcontext;
    state->tapeset = NULL;

    state->memtupcount = 0;
    state->memtupsize = 1024;   /* initial guess */
    state->growmemtuples = true;
    state->memtuples = (SortTuple *) palloc(state->memtupsize * sizeof(SortTuple));

    USEMEM(state, GetMemoryChunkSpace(state->memtuples));

    /* workMem must be large enough for the minimal memtuples array */
    if (LACKMEM(state))
        elog(ERROR, "insufficient memory allowed for sort");

    state->currentRun = 0;

    /*
     * maxTapes, tapeRange, and Algorithm D variables will be initialized by
     * inittapes(), if needed
     */

    state->result_tape = -1;    /* flag that result tape has not been formed */

    MemoryContextSwitchTo(oldcontext);

    return state;
}

Tuplesortstate *
tuplesort_begin_heap(TupleDesc tupDesc,
                     int nkeys, AttrNumber *attNums,
                     Oid *sortOperators, Oid *sortCollations,
                     bool *nullsFirstFlags,
                     int workMem, bool randomAccess)
{
    Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
    MemoryContext oldcontext;
    int         i;

    oldcontext = MemoryContextSwitchTo(state->sortcontext);

    AssertArg(nkeys > 0);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG,
             "begin tuple sort: nkeys = %d, workMem = %d, randomAccess = %c",
             nkeys, workMem, randomAccess ? 't' : 'f');
#endif

    state->nKeys = nkeys;

    TRACE_POSTGRESQL_SORT_START(HEAP_SORT,
                                false,  /* no unique check */
                                nkeys,
                                workMem,
                                randomAccess);

    state->comparetup = comparetup_heap;
    state->copytup = copytup_heap;
    state->writetup = writetup_heap;
    state->readtup = readtup_heap;
    state->reversedirection = reversedirection_heap;

    state->tupDesc = tupDesc;   /* assume we need not copy tupDesc */

    /* Prepare SortSupport data for each column */
    state->sortKeys = (SortSupport) palloc0(nkeys * sizeof(SortSupportData));

    for (i = 0; i < nkeys; i++)
    {
        SortSupport sortKey = state->sortKeys + i;

        AssertArg(attNums[i] != 0);
        AssertArg(sortOperators[i] != 0);

        sortKey->ssup_cxt = CurrentMemoryContext;
        sortKey->ssup_collation = sortCollations[i];
        sortKey->ssup_nulls_first = nullsFirstFlags[i];
        sortKey->ssup_attno = attNums[i];

        PrepareSortSupportFromOrderingOp(sortOperators[i], sortKey);
    }

    if (nkeys == 1)
        state->onlyKey = state->sortKeys;

    MemoryContextSwitchTo(oldcontext);

    return state;
}

Tuplesortstate *
tuplesort_begin_cluster(TupleDesc tupDesc,
                        Relation indexRel,
                        int workMem, bool randomAccess)
{
    Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
    MemoryContext oldcontext;

    Assert(indexRel->rd_rel->relam == BTREE_AM_OID);

    oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG,
             "begin tuple sort: nkeys = %d, workMem = %d, randomAccess = %c",
             RelationGetNumberOfAttributes(indexRel),
             workMem, randomAccess ? 't' : 'f');
#endif

    state->nKeys = RelationGetNumberOfAttributes(indexRel);

    TRACE_POSTGRESQL_SORT_START(CLUSTER_SORT,
                                false,  /* no unique check */
                                state->nKeys,
                                workMem,
                                randomAccess);

    state->comparetup = comparetup_cluster;
    state->copytup = copytup_cluster;
    state->writetup = writetup_cluster;
    state->readtup = readtup_cluster;
    state->reversedirection = reversedirection_index_btree;

    state->indexInfo = BuildIndexInfo(indexRel);
    state->indexScanKey = _bt_mkscankey_nodata(indexRel);

    state->tupDesc = tupDesc;   /* assume we need not copy tupDesc */

    if (state->indexInfo->ii_Expressions != NULL)
    {
        TupleTableSlot *slot;
        ExprContext *econtext;

        /*
         * We will need to use FormIndexDatum to evaluate the index
         * expressions.  To do that, we need an EState, as well as a
         * TupleTableSlot to put the table tuples into.  The econtext's
         * scantuple has to point to that slot, too.
         */
        state->estate = CreateExecutorState();
        slot = MakeSingleTupleTableSlot(tupDesc);
        econtext = GetPerTupleExprContext(state->estate);
        econtext->ecxt_scantuple = slot;
    }

    MemoryContextSwitchTo(oldcontext);

    return state;
}

Tuplesortstate *
tuplesort_begin_index_btree(Relation heapRel,
                            Relation indexRel,
                            bool enforceUnique,
                            int workMem, bool randomAccess)
{
    Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
    MemoryContext oldcontext;

    oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG,
             "begin index sort: unique = %c, workMem = %d, randomAccess = %c",
             enforceUnique ? 't' : 'f',
             workMem, randomAccess ? 't' : 'f');
#endif

    state->nKeys = RelationGetNumberOfAttributes(indexRel);

    TRACE_POSTGRESQL_SORT_START(INDEX_SORT,
                                enforceUnique,
                                state->nKeys,
                                workMem,
                                randomAccess);

    state->comparetup = comparetup_index_btree;
    state->copytup = copytup_index;
    state->writetup = writetup_index;
    state->readtup = readtup_index;
    state->reversedirection = reversedirection_index_btree;

    state->heapRel = heapRel;
    state->indexRel = indexRel;
    state->indexScanKey = _bt_mkscankey_nodata(indexRel);
    state->enforceUnique = enforceUnique;

    MemoryContextSwitchTo(oldcontext);

    return state;
}

Tuplesortstate *
tuplesort_begin_index_hash(Relation heapRel,
                           Relation indexRel,
                           uint32 hash_mask,
                           int workMem, bool randomAccess)
{
    Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
    MemoryContext oldcontext;

    oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG,
        "begin index sort: hash_mask = 0x%x, workMem = %d, randomAccess = %c",
             hash_mask,
             workMem, randomAccess ? 't' : 'f');
#endif

    state->nKeys = 1;           /* Only one sort column, the hash code */

    state->comparetup = comparetup_index_hash;
    state->copytup = copytup_index;
    state->writetup = writetup_index;
    state->readtup = readtup_index;
    state->reversedirection = reversedirection_index_hash;

    state->heapRel = heapRel;
    state->indexRel = indexRel;

    state->hash_mask = hash_mask;

    MemoryContextSwitchTo(oldcontext);

    return state;
}

Tuplesortstate *
tuplesort_begin_datum(Oid datumType, Oid sortOperator, Oid sortCollation,
                      bool nullsFirstFlag,
                      int workMem, bool randomAccess)
{
    Tuplesortstate *state = tuplesort_begin_common(workMem, randomAccess);
    MemoryContext oldcontext;
    int16       typlen;
    bool        typbyval;

    oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG,
             "begin datum sort: workMem = %d, randomAccess = %c",
             workMem, randomAccess ? 't' : 'f');
#endif

    state->nKeys = 1;           /* always a one-column sort */

    TRACE_POSTGRESQL_SORT_START(DATUM_SORT,
                                false,  /* no unique check */
                                1,
                                workMem,
                                randomAccess);

    state->comparetup = comparetup_datum;
    state->copytup = copytup_datum;
    state->writetup = writetup_datum;
    state->readtup = readtup_datum;
    state->reversedirection = reversedirection_datum;

    state->datumType = datumType;

    /* Prepare SortSupport data */
    state->onlyKey = (SortSupport) palloc0(sizeof(SortSupportData));

    state->onlyKey->ssup_cxt = CurrentMemoryContext;
    state->onlyKey->ssup_collation = sortCollation;
    state->onlyKey->ssup_nulls_first = nullsFirstFlag;

    PrepareSortSupportFromOrderingOp(sortOperator, state->onlyKey);

    /* lookup necessary attributes of the datum type */
    get_typlenbyval(datumType, &typlen, &typbyval);
    state->datumTypeLen = typlen;
    state->datumTypeByVal = typbyval;

    MemoryContextSwitchTo(oldcontext);

    return state;
}

/*
 * tuplesort_set_bound
 *
 *  Advise tuplesort that at most the first N result tuples are required.
 *
 * Must be called before inserting any tuples.  (Actually, we could allow it
 * as long as the sort hasn't spilled to disk, but there seems no need for
 * delayed calls at the moment.)
 *
 * This is a hint only. The tuplesort may still return more tuples than
 * requested.
 */
void
tuplesort_set_bound(Tuplesortstate *state, int64 bound)
{
    /* Assert we're called before loading any tuples */
    Assert(state->status == TSS_INITIAL);
    Assert(state->memtupcount == 0);
    Assert(!state->bounded);

#ifdef DEBUG_BOUNDED_SORT
    /* Honor GUC setting that disables the feature (for easy testing) */
    if (!optimize_bounded_sort)
        return;
#endif

    /* We want to be able to compute bound * 2, so limit the setting */
    if (bound > (int64) (INT_MAX / 2))
        return;

    state->bounded = true;
    state->bound = (int) bound;
}

/*
 * tuplesort_end
 *
 *  Release resources and clean up.
 *
 * NOTE: after calling this, any pointers returned by tuplesort_getXXX are
 * pointing to garbage.  Be careful not to attempt to use or free such
 * pointers afterwards!
 */
void
tuplesort_end(Tuplesortstate *state)
{
    /* context swap probably not needed, but let's be safe */
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    long        spaceUsed;

    if (state->tapeset)
        spaceUsed = LogicalTapeSetBlocks(state->tapeset);
    else
        spaceUsed = (state->allowedMem - state->availMem + 1023) / 1024;
#endif

    /*
     * Delete temporary "tape" files, if any.
     *
     * Note: want to include this in reported total cost of sort, hence need
     * for two #ifdef TRACE_SORT sections.
     */
    if (state->tapeset)
        LogicalTapeSetClose(state->tapeset);

#ifdef TRACE_SORT
    if (trace_sort)
    {
        if (state->tapeset)
            elog(LOG, "external sort ended, %ld disk blocks used: %s",
                 spaceUsed, pg_rusage_show(&state->ru_start));
        else
            elog(LOG, "internal sort ended, %ld KB used: %s",
                 spaceUsed, pg_rusage_show(&state->ru_start));
    }

    TRACE_POSTGRESQL_SORT_DONE(state->tapeset != NULL, spaceUsed);
#else

    /*
     * If you disabled TRACE_SORT, you can still probe sort__done, but you
     * ain't getting space-used stats.
     */
    TRACE_POSTGRESQL_SORT_DONE(state->tapeset != NULL, 0L);
#endif

    /* Free any execution state created for CLUSTER case */
    if (state->estate != NULL)
    {
        ExprContext *econtext = GetPerTupleExprContext(state->estate);

        ExecDropSingleTupleTableSlot(econtext->ecxt_scantuple);
        FreeExecutorState(state->estate);
    }

    MemoryContextSwitchTo(oldcontext);

    /*
     * Free the per-sort memory context, thereby releasing all working memory,
     * including the Tuplesortstate struct itself.
     */
    MemoryContextDelete(state->sortcontext);
}

/*
 * Grow the memtuples[] array, if possible within our memory constraint.
 * Return TRUE if we were able to enlarge the array, FALSE if not.
 *
 * Normally, at each increment we double the size of the array.  When we no
 * longer have enough memory to do that, we attempt one last, smaller increase
 * (and then clear the growmemtuples flag so we don't try any more).  That
 * allows us to use allowedMem as fully as possible; sticking to the pure
 * doubling rule could result in almost half of allowedMem going unused.
 * Because availMem moves around with tuple addition/removal, we need some
 * rule to prevent making repeated small increases in memtupsize, which would
 * just be useless thrashing.  The growmemtuples flag accomplishes that and
 * also prevents useless recalculations in this function.
 */
static bool
grow_memtuples(Tuplesortstate *state)
{
    int         newmemtupsize;
    int         memtupsize = state->memtupsize;
    long        memNowUsed = state->allowedMem - state->availMem;

    /* Forget it if we've already maxed out memtuples, per comment above */
    if (!state->growmemtuples)
        return false;

    /* Select new value of memtupsize */
    if (memNowUsed <= state->availMem)
    {
        /*
         * It is surely safe to double memtupsize if we've used no more than
         * half of allowedMem.
         *
         * Note: it might seem that we need to worry about memtupsize * 2
         * overflowing an int, but the MaxAllocSize clamp applied below
         * ensures the existing memtupsize can't be large enough for that.
         */
        newmemtupsize = memtupsize * 2;
    }
    else
    {
        /*
         * This will be the last increment of memtupsize.  Abandon doubling
         * strategy and instead increase as much as we safely can.
         *
         * To stay within allowedMem, we can't increase memtupsize by more
         * than availMem / sizeof(SortTuple) elements.  In practice, we want
         * to increase it by considerably less, because we need to leave some
         * space for the tuples to which the new array slots will refer.  We
         * assume the new tuples will be about the same size as the tuples
         * we've already seen, and thus we can extrapolate from the space
         * consumption so far to estimate an appropriate new size for the
         * memtuples array.  The optimal value might be higher or lower than
         * this estimate, but it's hard to know that in advance.
         *
         * This calculation is safe against enlarging the array so much that
         * LACKMEM becomes true, because the memory currently used includes
         * the present array; thus, there would be enough allowedMem for the
         * new array elements even if no other memory were currently used.
         *
         * We do the arithmetic in float8, because otherwise the product of
         * memtupsize and allowedMem could overflow.  (A little algebra shows
         * that grow_ratio must be less than 2 here, so we are not risking
         * integer overflow this way.)  Any inaccuracy in the result should be
         * insignificant; but even if we computed a completely insane result,
         * the checks below will prevent anything really bad from happening.
         */
        double      grow_ratio;

        grow_ratio = (double) state->allowedMem / (double) memNowUsed;
        newmemtupsize = (int) (memtupsize * grow_ratio);

        /* We won't make any further enlargement attempts */
        state->growmemtuples = false;
    }

    /* Must enlarge array by at least one element, else report failure */
    if (newmemtupsize <= memtupsize)
        goto noalloc;

    /*
     * On a 64-bit machine, allowedMem could be more than MaxAllocSize.  Clamp
     * to ensure our request won't be rejected by palloc.
     */
    if ((Size) newmemtupsize >= MaxAllocSize / sizeof(SortTuple))
    {
        newmemtupsize = (int) (MaxAllocSize / sizeof(SortTuple));
        state->growmemtuples = false;   /* can't grow any more */
    }

    /*
     * We need to be sure that we do not cause LACKMEM to become true, else
     * the space management algorithm will go nuts.  The code above should
     * never generate a dangerous request, but to be safe, check explicitly
     * that the array growth fits within availMem.  (We could still cause
     * LACKMEM if the memory chunk overhead associated with the memtuples
     * array were to increase.  That shouldn't happen with any sane value of
     * allowedMem, because at any array size large enough to risk LACKMEM,
     * palloc would be treating both old and new arrays as separate chunks.
     * But we'll check LACKMEM explicitly below just in case.)
     */
    if (state->availMem < (long) ((newmemtupsize - memtupsize) * sizeof(SortTuple)))
        goto noalloc;

    /* OK, do it */
    FREEMEM(state, GetMemoryChunkSpace(state->memtuples));
    state->memtupsize = newmemtupsize;
    state->memtuples = (SortTuple *)
        repalloc(state->memtuples,
                 state->memtupsize * sizeof(SortTuple));
    USEMEM(state, GetMemoryChunkSpace(state->memtuples));
    if (LACKMEM(state))
        elog(ERROR, "unexpected out-of-memory situation during sort");
    return true;

noalloc:
    /* If for any reason we didn't realloc, shut off future attempts */
    state->growmemtuples = false;
    return false;
}

/*
 * Accept one tuple while collecting input data for sort.
 *
 * Note that the input data is always copied; the caller need not save it.
 */
void
tuplesort_puttupleslot(Tuplesortstate *state, TupleTableSlot *slot)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    /*
     * Copy the given tuple into memory we control, and decrease availMem.
     * Then call the common code.
     */
    COPYTUP(state, &stup, (void *) slot);

    puttuple_common(state, &stup);

    MemoryContextSwitchTo(oldcontext);
}

/*
 * Accept one tuple while collecting input data for sort.
 *
 * Note that the input data is always copied; the caller need not save it.
 */
void
tuplesort_putheaptuple(Tuplesortstate *state, HeapTuple tup)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    /*
     * Copy the given tuple into memory we control, and decrease availMem.
     * Then call the common code.
     */
    COPYTUP(state, &stup, (void *) tup);

    puttuple_common(state, &stup);

    MemoryContextSwitchTo(oldcontext);
}

/*
 * Accept one index tuple while collecting input data for sort.
 *
 * Note that the input tuple is always copied; the caller need not save it.
 */
void
tuplesort_putindextuple(Tuplesortstate *state, IndexTuple tuple)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    /*
     * Copy the given tuple into memory we control, and decrease availMem.
     * Then call the common code.
     */
    COPYTUP(state, &stup, (void *) tuple);

    puttuple_common(state, &stup);

    MemoryContextSwitchTo(oldcontext);
}

/*
 * Accept one Datum while collecting input data for sort.
 *
 * If the Datum is pass-by-ref type, the value will be copied.
 */
void
tuplesort_putdatum(Tuplesortstate *state, Datum val, bool isNull)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    /*
     * If it's a pass-by-reference value, copy it into memory we control, and
     * decrease availMem.  Then call the common code.
     */
    if (isNull || state->datumTypeByVal)
    {
        stup.datum1 = val;
        stup.isnull1 = isNull;
        stup.tuple = NULL;      /* no separate storage */
    }
    else
    {
        stup.datum1 = datumCopy(val, false, state->datumTypeLen);
        stup.isnull1 = false;
        stup.tuple = DatumGetPointer(stup.datum1);
        USEMEM(state, GetMemoryChunkSpace(stup.tuple));
    }

    puttuple_common(state, &stup);

    MemoryContextSwitchTo(oldcontext);
}

/*
 * Shared code for tuple and datum cases.
 */
static void
puttuple_common(Tuplesortstate *state, SortTuple *tuple)
{
    switch (state->status)
    {
        case TSS_INITIAL:

            /*
             * Save the tuple into the unsorted array.  First, grow the array
             * as needed.  Note that we try to grow the array when there is
             * still one free slot remaining --- if we fail, there'll still be
             * room to store the incoming tuple, and then we'll switch to
             * tape-based operation.
             */
            if (state->memtupcount >= state->memtupsize - 1)
            {
                (void) grow_memtuples(state);
                Assert(state->memtupcount < state->memtupsize);
            }
            state->memtuples[state->memtupcount++] = *tuple;

            /*
             * Check if it's time to switch over to a bounded heapsort. We do
             * so if the input tuple count exceeds twice the desired tuple
             * count (this is a heuristic for where heapsort becomes cheaper
             * than a quicksort), or if we've just filled workMem and have
             * enough tuples to meet the bound.
             *
             * Note that once we enter TSS_BOUNDED state we will always try to
             * complete the sort that way.  In the worst case, if later input
             * tuples are larger than earlier ones, this might cause us to
             * exceed workMem significantly.
             */
            if (state->bounded &&
                (state->memtupcount > state->bound * 2 ||
                 (state->memtupcount > state->bound && LACKMEM(state))))
            {
#ifdef TRACE_SORT
                if (trace_sort)
                    elog(LOG, "switching to bounded heapsort at %d tuples: %s",
                         state->memtupcount,
                         pg_rusage_show(&state->ru_start));
#endif
                make_bounded_heap(state);
                return;
            }

            /*
             * Done if we still fit in available memory and have array slots.
             */
            if (state->memtupcount < state->memtupsize && !LACKMEM(state))
                return;

            /*
             * Nope; time to switch to tape-based operation.
             */
            inittapes(state);

            /*
             * Dump tuples until we are back under the limit.
             */
            dumptuples(state, false);
            break;

        case TSS_BOUNDED:

            /*
             * We don't want to grow the array here, so check whether the new
             * tuple can be discarded before putting it in.  This should be a
             * good speed optimization, too, since when there are many more
             * input tuples than the bound, most input tuples can be discarded
             * with just this one comparison.  Note that because we currently
             * have the sort direction reversed, we must check for <= not >=.
             */
            if (COMPARETUP(state, tuple, &state->memtuples[0]) <= 0)
            {
                /* new tuple <= top of the heap, so we can discard it */
                free_sort_tuple(state, tuple);
                CHECK_FOR_INTERRUPTS();
            }
            else
            {
                /* discard top of heap, sift up, insert new tuple */
                free_sort_tuple(state, &state->memtuples[0]);
                tuplesort_heap_siftup(state, false);
                tuplesort_heap_insert(state, tuple, 0, false);
            }
            break;

        case TSS_BUILDRUNS:

            /*
             * Insert the tuple into the heap, with run number currentRun if
             * it can go into the current run, else run number currentRun+1.
             * The tuple can go into the current run if it is >= the first
             * not-yet-output tuple.  (Actually, it could go into the current
             * run if it is >= the most recently output tuple ... but that
             * would require keeping around the tuple we last output, and it's
             * simplest to let writetup free each tuple as soon as it's
             * written.)
             *
             * Note there will always be at least one tuple in the heap at
             * this point; see dumptuples.
             */
            Assert(state->memtupcount > 0);
            if (COMPARETUP(state, tuple, &state->memtuples[0]) >= 0)
                tuplesort_heap_insert(state, tuple, state->currentRun, true);
            else
                tuplesort_heap_insert(state, tuple, state->currentRun + 1, true);

            /*
             * If we are over the memory limit, dump tuples till we're under.
             */
            dumptuples(state, false);
            break;

        default:
            elog(ERROR, "invalid tuplesort state");
            break;
    }
}

/*
 * All tuples have been provided; finish the sort.
 */
void
tuplesort_performsort(Tuplesortstate *state)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG, "performsort starting: %s",
             pg_rusage_show(&state->ru_start));
#endif

    switch (state->status)
    {
        case TSS_INITIAL:

            /*
             * We were able to accumulate all the tuples within the allowed
             * amount of memory.  Just qsort 'em and we're done.
             */
            if (state->memtupcount > 1)
            {
                /* Can we use the single-key sort function? */
                if (state->onlyKey != NULL)
                    qsort_ssup(state->memtuples, state->memtupcount,
                               state->onlyKey);
                else
                    qsort_tuple(state->memtuples,
                                state->memtupcount,
                                state->comparetup,
                                state);
            }
            state->current = 0;
            state->eof_reached = false;
            state->markpos_offset = 0;
            state->markpos_eof = false;
            state->status = TSS_SORTEDINMEM;
            break;

        case TSS_BOUNDED:

            /*
             * We were able to accumulate all the tuples required for output
             * in memory, using a heap to eliminate excess tuples.  Now we
             * have to transform the heap to a properly-sorted array.
             */
            sort_bounded_heap(state);
            state->current = 0;
            state->eof_reached = false;
            state->markpos_offset = 0;
            state->markpos_eof = false;
            state->status = TSS_SORTEDINMEM;
            break;

        case TSS_BUILDRUNS:

            /*
             * Finish tape-based sort.  First, flush all tuples remaining in
             * memory out to tape; then merge until we have a single remaining
             * run (or, if !randomAccess, one run per tape). Note that
             * mergeruns sets the correct state->status.
             */
            dumptuples(state, true);
            mergeruns(state);
            state->eof_reached = false;
            state->markpos_block = 0L;
            state->markpos_offset = 0;
            state->markpos_eof = false;
            break;

        default:
            elog(ERROR, "invalid tuplesort state");
            break;
    }

#ifdef TRACE_SORT
    if (trace_sort)
    {
        if (state->status == TSS_FINALMERGE)
            elog(LOG, "performsort done (except %d-way final merge): %s",
                 state->activeTapes,
                 pg_rusage_show(&state->ru_start));
        else
            elog(LOG, "performsort done: %s",
                 pg_rusage_show(&state->ru_start));
    }
#endif

    MemoryContextSwitchTo(oldcontext);
}

/*
 * Internal routine to fetch the next tuple in either forward or back
 * direction into *stup.  Returns FALSE if no more tuples.
 * If *should_free is set, the caller must pfree stup.tuple when done with it.
 */
static bool
tuplesort_gettuple_common(Tuplesortstate *state, bool forward,
                          SortTuple *stup, bool *should_free)
{
    unsigned int tuplen;

    switch (state->status)
    {
        case TSS_SORTEDINMEM:
            Assert(forward || state->randomAccess);
            *should_free = false;
            if (forward)
            {
                if (state->current < state->memtupcount)
                {
                    *stup = state->memtuples[state->current++];
                    return true;
                }
                state->eof_reached = true;

                /*
                 * Complain if caller tries to retrieve more tuples than
                 * originally asked for in a bounded sort.  This is because
                 * returning EOF here might be the wrong thing.
                 */
                if (state->bounded && state->current >= state->bound)
                    elog(ERROR, "retrieved too many tuples in a bounded sort");

                return false;
            }
            else
            {
                if (state->current <= 0)
                    return false;

                /*
                 * if all tuples are fetched already then we return last
                 * tuple, else - tuple before last returned.
                 */
                if (state->eof_reached)
                    state->eof_reached = false;
                else
                {
                    state->current--;   /* last returned tuple */
                    if (state->current <= 0)
                        return false;
                }
                *stup = state->memtuples[state->current - 1];
                return true;
            }
            break;

        case TSS_SORTEDONTAPE:
            Assert(forward || state->randomAccess);
            *should_free = true;
            if (forward)
            {
                if (state->eof_reached)
                    return false;
                if ((tuplen = getlen(state, state->result_tape, true)) != 0)
                {
                    READTUP(state, stup, state->result_tape, tuplen);
                    return true;
                }
                else
                {
                    state->eof_reached = true;
                    return false;
                }
            }

            /*
             * Backward.
             *
             * if all tuples are fetched already then we return last tuple,
             * else - tuple before last returned.
             */
            if (state->eof_reached)
            {
                /*
                 * Seek position is pointing just past the zero tuplen at the
                 * end of file; back up to fetch last tuple's ending length
                 * word.  If seek fails we must have a completely empty file.
                 */
                if (!LogicalTapeBackspace(state->tapeset,
                                          state->result_tape,
                                          2 * sizeof(unsigned int)))
                    return false;
                state->eof_reached = false;
            }
            else
            {
                /*
                 * Back up and fetch previously-returned tuple's ending length
                 * word.  If seek fails, assume we are at start of file.
                 */
                if (!LogicalTapeBackspace(state->tapeset,
                                          state->result_tape,
                                          sizeof(unsigned int)))
                    return false;
                tuplen = getlen(state, state->result_tape, false);

                /*
                 * Back up to get ending length word of tuple before it.
                 */
                if (!LogicalTapeBackspace(state->tapeset,
                                          state->result_tape,
                                          tuplen + 2 * sizeof(unsigned int)))
                {
                    /*
                     * If that fails, presumably the prev tuple is the first
                     * in the file.  Back up so that it becomes next to read
                     * in forward direction (not obviously right, but that is
                     * what in-memory case does).
                     */
                    if (!LogicalTapeBackspace(state->tapeset,
                                              state->result_tape,
                                              tuplen + sizeof(unsigned int)))
                        elog(ERROR, "bogus tuple length in backward scan");
                    return false;
                }
            }

            tuplen = getlen(state, state->result_tape, false);

            /*
             * Now we have the length of the prior tuple, back up and read it.
             * Note: READTUP expects we are positioned after the initial
             * length word of the tuple, so back up to that point.
             */
            if (!LogicalTapeBackspace(state->tapeset,
                                      state->result_tape,
                                      tuplen))
                elog(ERROR, "bogus tuple length in backward scan");
            READTUP(state, stup, state->result_tape, tuplen);
            return true;

        case TSS_FINALMERGE:
            Assert(forward);
            *should_free = true;

            /*
             * This code should match the inner loop of mergeonerun().
             */
            if (state->memtupcount > 0)
            {
                int         srcTape = state->memtuples[0].tupindex;
                Size        tuplen;
                int         tupIndex;
                SortTuple  *newtup;

                *stup = state->memtuples[0];
                /* returned tuple is no longer counted in our memory space */
                if (stup->tuple)
                {
                    tuplen = GetMemoryChunkSpace(stup->tuple);
                    state->availMem += tuplen;
                    state->mergeavailmem[srcTape] += tuplen;
                }
                tuplesort_heap_siftup(state, false);
                if ((tupIndex = state->mergenext[srcTape]) == 0)
                {
                    /*
                     * out of preloaded data on this tape, try to read more
                     *
                     * Unlike mergeonerun(), we only preload from the single
                     * tape that's run dry.  See mergepreread() comments.
                     */
                    mergeprereadone(state, srcTape);

                    /*
                     * if still no data, we've reached end of run on this tape
                     */
                    if ((tupIndex = state->mergenext[srcTape]) == 0)
                        return true;
                }
                /* pull next preread tuple from list, insert in heap */
                newtup = &state->memtuples[tupIndex];
                state->mergenext[srcTape] = newtup->tupindex;
                if (state->mergenext[srcTape] == 0)
                    state->mergelast[srcTape] = 0;
                tuplesort_heap_insert(state, newtup, srcTape, false);
                /* put the now-unused memtuples entry on the freelist */
                newtup->tupindex = state->mergefreelist;
                state->mergefreelist = tupIndex;
                state->mergeavailslots[srcTape]++;
                return true;
            }
            return false;

        default:
            elog(ERROR, "invalid tuplesort state");
            return false;       /* keep compiler quiet */
    }
}

/*
 * Fetch the next tuple in either forward or back direction.
 * If successful, put tuple in slot and return TRUE; else, clear the slot
 * and return FALSE.
 */
bool
tuplesort_gettupleslot(Tuplesortstate *state, bool forward,
                       TupleTableSlot *slot)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;
    bool        should_free;

    if (!tuplesort_gettuple_common(state, forward, &stup, &should_free))
        stup.tuple = NULL;

    MemoryContextSwitchTo(oldcontext);

    if (stup.tuple)
    {
        ExecStoreMinimalTuple((MinimalTuple) stup.tuple, slot, should_free);
        return true;
    }
    else
    {
        ExecClearTuple(slot);
        return false;
    }
}

/*
 * Fetch the next tuple in either forward or back direction.
 * Returns NULL if no more tuples.  If *should_free is set, the
 * caller must pfree the returned tuple when done with it.
 */
HeapTuple
tuplesort_getheaptuple(Tuplesortstate *state, bool forward, bool *should_free)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    if (!tuplesort_gettuple_common(state, forward, &stup, should_free))
        stup.tuple = NULL;

    MemoryContextSwitchTo(oldcontext);

    return stup.tuple;
}

/*
 * Fetch the next index tuple in either forward or back direction.
 * Returns NULL if no more tuples.  If *should_free is set, the
 * caller must pfree the returned tuple when done with it.
 */
IndexTuple
tuplesort_getindextuple(Tuplesortstate *state, bool forward,
                        bool *should_free)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;

    if (!tuplesort_gettuple_common(state, forward, &stup, should_free))
        stup.tuple = NULL;

    MemoryContextSwitchTo(oldcontext);

    return (IndexTuple) stup.tuple;
}

/*
 * Fetch the next Datum in either forward or back direction.
 * Returns FALSE if no more datums.
 *
 * If the Datum is pass-by-ref type, the returned value is freshly palloc'd
 * and is now owned by the caller.
 */
bool
tuplesort_getdatum(Tuplesortstate *state, bool forward,
                   Datum *val, bool *isNull)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);
    SortTuple   stup;
    bool        should_free;

    if (!tuplesort_gettuple_common(state, forward, &stup, &should_free))
    {
        MemoryContextSwitchTo(oldcontext);
        return false;
    }

    if (stup.isnull1 || state->datumTypeByVal)
    {
        *val = stup.datum1;
        *isNull = stup.isnull1;
    }
    else
    {
        if (should_free)
            *val = stup.datum1;
        else
            *val = datumCopy(stup.datum1, false, state->datumTypeLen);
        *isNull = false;
    }

    MemoryContextSwitchTo(oldcontext);

    return true;
}

/*
 * tuplesort_merge_order - report merge order we'll use for given memory
 * (note: "merge order" just means the number of input tapes in the merge).
 *
 * This is exported for use by the planner.  allowedMem is in bytes.
 */
int
tuplesort_merge_order(long allowedMem)
{
    int         mOrder;

    /*
     * We need one tape for each merge input, plus another one for the output,
     * and each of these tapes needs buffer space.  In addition we want
     * MERGE_BUFFER_SIZE workspace per input tape (but the output tape doesn't
     * count).
     *
     * Note: you might be thinking we need to account for the memtuples[]
     * array in this calculation, but we effectively treat that as part of the
     * MERGE_BUFFER_SIZE workspace.
     */
    mOrder = (allowedMem - TAPE_BUFFER_OVERHEAD) /
        (MERGE_BUFFER_SIZE + TAPE_BUFFER_OVERHEAD);

    /* Even in minimum memory, use at least a MINORDER merge */
    mOrder = Max(mOrder, MINORDER);

    return mOrder;
}

/*
 * inittapes - initialize for tape sorting.
 *
 * This is called only if we have found we don't have room to sort in memory.
 */
static void
inittapes(Tuplesortstate *state)
{
    int         maxTapes,
                ntuples,
                j;
    long        tapeSpace;

    /* Compute number of tapes to use: merge order plus 1 */
    maxTapes = tuplesort_merge_order(state->allowedMem) + 1;

    /*
     * We must have at least 2*maxTapes slots in the memtuples[] array, else
     * we'd not have room for merge heap plus preread.  It seems unlikely that
     * this case would ever occur, but be safe.
     */
    maxTapes = Min(maxTapes, state->memtupsize / 2);

    state->maxTapes = maxTapes;
    state->tapeRange = maxTapes - 1;

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG, "switching to external sort with %d tapes: %s",
             maxTapes, pg_rusage_show(&state->ru_start));
#endif

    /*
     * Decrease availMem to reflect the space needed for tape buffers; but
     * don't decrease it to the point that we have no room for tuples. (That
     * case is only likely to occur if sorting pass-by-value Datums; in all
     * other scenarios the memtuples[] array is unlikely to occupy more than
     * half of allowedMem.  In the pass-by-value case it's not important to
     * account for tuple space, so we don't care if LACKMEM becomes
     * inaccurate.)
     */
    tapeSpace = maxTapes * TAPE_BUFFER_OVERHEAD;
    if (tapeSpace + GetMemoryChunkSpace(state->memtuples) < state->allowedMem)
        USEMEM(state, tapeSpace);

    /*
     * Make sure that the temp file(s) underlying the tape set are created in
     * suitable temp tablespaces.
     */
    PrepareTempTablespaces();

    /*
     * Create the tape set and allocate the per-tape data arrays.
     */
    state->tapeset = LogicalTapeSetCreate(maxTapes);

    state->mergeactive = (bool *) palloc0(maxTapes * sizeof(bool));
    state->mergenext = (int *) palloc0(maxTapes * sizeof(int));
    state->mergelast = (int *) palloc0(maxTapes * sizeof(int));
    state->mergeavailslots = (int *) palloc0(maxTapes * sizeof(int));
    state->mergeavailmem = (long *) palloc0(maxTapes * sizeof(long));
    state->tp_fib = (int *) palloc0(maxTapes * sizeof(int));
    state->tp_runs = (int *) palloc0(maxTapes * sizeof(int));
    state->tp_dummy = (int *) palloc0(maxTapes * sizeof(int));
    state->tp_tapenum = (int *) palloc0(maxTapes * sizeof(int));

    /*
     * Convert the unsorted contents of memtuples[] into a heap. Each tuple is
     * marked as belonging to run number zero.
     *
     * NOTE: we pass false for checkIndex since there's no point in comparing
     * indexes in this step, even though we do intend the indexes to be part
     * of the sort key...
     */
    ntuples = state->memtupcount;
    state->memtupcount = 0;     /* make the heap empty */
    for (j = 0; j < ntuples; j++)
    {
        /* Must copy source tuple to avoid possible overwrite */
        SortTuple   stup = state->memtuples[j];

        tuplesort_heap_insert(state, &stup, 0, false);
    }
    Assert(state->memtupcount == ntuples);

    state->currentRun = 0;

    /*
     * Initialize variables of Algorithm D (step D1).
     */
    for (j = 0; j < maxTapes; j++)
    {
        state->tp_fib[j] = 1;
        state->tp_runs[j] = 0;
        state->tp_dummy[j] = 1;
        state->tp_tapenum[j] = j;
    }
    state->tp_fib[state->tapeRange] = 0;
    state->tp_dummy[state->tapeRange] = 0;

    state->Level = 1;
    state->destTape = 0;

    state->status = TSS_BUILDRUNS;
}

/*
 * selectnewtape -- select new tape for new initial run.
 *
 * This is called after finishing a run when we know another run
 * must be started.  This implements steps D3, D4 of Algorithm D.
 */
static void
selectnewtape(Tuplesortstate *state)
{
    int         j;
    int         a;

    /* Step D3: advance j (destTape) */
    if (state->tp_dummy[state->destTape] < state->tp_dummy[state->destTape + 1])
    {
        state->destTape++;
        return;
    }
    if (state->tp_dummy[state->destTape] != 0)
    {
        state->destTape = 0;
        return;
    }

    /* Step D4: increase level */
    state->Level++;
    a = state->tp_fib[0];
    for (j = 0; j < state->tapeRange; j++)
    {
        state->tp_dummy[j] = a + state->tp_fib[j + 1] - state->tp_fib[j];
        state->tp_fib[j] = a + state->tp_fib[j + 1];
    }
    state->destTape = 0;
}

/*
 * mergeruns -- merge all the completed initial runs.
 *
 * This implements steps D5, D6 of Algorithm D.  All input data has
 * already been written to initial runs on tape (see dumptuples).
 */
static void
mergeruns(Tuplesortstate *state)
{
    int         tapenum,
                svTape,
                svRuns,
                svDummy;

    Assert(state->status == TSS_BUILDRUNS);
    Assert(state->memtupcount == 0);

    /*
     * If we produced only one initial run (quite likely if the total data
     * volume is between 1X and 2X workMem), we can just use that tape as the
     * finished output, rather than doing a useless merge.  (This obvious
     * optimization is not in Knuth's algorithm.)
     */
    if (state->currentRun == 1)
    {
        state->result_tape = state->tp_tapenum[state->destTape];
        /* must freeze and rewind the finished output tape */
        LogicalTapeFreeze(state->tapeset, state->result_tape);
        state->status = TSS_SORTEDONTAPE;
        return;
    }

    /* End of step D2: rewind all output tapes to prepare for merging */
    for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
        LogicalTapeRewind(state->tapeset, tapenum, false);

    for (;;)
    {
        /*
         * At this point we know that tape[T] is empty.  If there's just one
         * (real or dummy) run left on each input tape, then only one merge
         * pass remains.  If we don't have to produce a materialized sorted
         * tape, we can stop at this point and do the final merge on-the-fly.
         */
        if (!state->randomAccess)
        {
            bool        allOneRun = true;

            Assert(state->tp_runs[state->tapeRange] == 0);
            for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
            {
                if (state->tp_runs[tapenum] + state->tp_dummy[tapenum] != 1)
                {
                    allOneRun = false;
                    break;
                }
            }
            if (allOneRun)
            {
                /* Tell logtape.c we won't be writing anymore */
                LogicalTapeSetForgetFreeSpace(state->tapeset);
                /* Initialize for the final merge pass */
                beginmerge(state);
                state->status = TSS_FINALMERGE;
                return;
            }
        }

        /* Step D5: merge runs onto tape[T] until tape[P] is empty */
        while (state->tp_runs[state->tapeRange - 1] ||
               state->tp_dummy[state->tapeRange - 1])
        {
            bool        allDummy = true;

            for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
            {
                if (state->tp_dummy[tapenum] == 0)
                {
                    allDummy = false;
                    break;
                }
            }

            if (allDummy)
            {
                state->tp_dummy[state->tapeRange]++;
                for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
                    state->tp_dummy[tapenum]--;
            }
            else
                mergeonerun(state);
        }

        /* Step D6: decrease level */
        if (--state->Level == 0)
            break;
        /* rewind output tape T to use as new input */
        LogicalTapeRewind(state->tapeset, state->tp_tapenum[state->tapeRange],
                          false);
        /* rewind used-up input tape P, and prepare it for write pass */
        LogicalTapeRewind(state->tapeset, state->tp_tapenum[state->tapeRange - 1],
                          true);
        state->tp_runs[state->tapeRange - 1] = 0;

        /*
         * reassign tape units per step D6; note we no longer care about A[]
         */
        svTape = state->tp_tapenum[state->tapeRange];
        svDummy = state->tp_dummy[state->tapeRange];
        svRuns = state->tp_runs[state->tapeRange];
        for (tapenum = state->tapeRange; tapenum > 0; tapenum--)
        {
            state->tp_tapenum[tapenum] = state->tp_tapenum[tapenum - 1];
            state->tp_dummy[tapenum] = state->tp_dummy[tapenum - 1];
            state->tp_runs[tapenum] = state->tp_runs[tapenum - 1];
        }
        state->tp_tapenum[0] = svTape;
        state->tp_dummy[0] = svDummy;
        state->tp_runs[0] = svRuns;
    }

    /*
     * Done.  Knuth says that the result is on TAPE[1], but since we exited
     * the loop without performing the last iteration of step D6, we have not
     * rearranged the tape unit assignment, and therefore the result is on
     * TAPE[T].  We need to do it this way so that we can freeze the final
     * output tape while rewinding it.  The last iteration of step D6 would be
     * a waste of cycles anyway...
     */
    state->result_tape = state->tp_tapenum[state->tapeRange];
    LogicalTapeFreeze(state->tapeset, state->result_tape);
    state->status = TSS_SORTEDONTAPE;
}

/*
 * Merge one run from each input tape, except ones with dummy runs.
 *
 * This is the inner loop of Algorithm D step D5.  We know that the
 * output tape is TAPE[T].
 */
static void
mergeonerun(Tuplesortstate *state)
{
    int         destTape = state->tp_tapenum[state->tapeRange];
    int         srcTape;
    int         tupIndex;
    SortTuple  *tup;
    long        priorAvail,
                spaceFreed;

    /*
     * Start the merge by loading one tuple from each active source tape into
     * the heap.  We can also decrease the input run/dummy run counts.
     */
    beginmerge(state);

    /*
     * Execute merge by repeatedly extracting lowest tuple in heap, writing it
     * out, and replacing it with next tuple from same tape (if there is
     * another one).
     */
    while (state->memtupcount > 0)
    {
        /* write the tuple to destTape */
        priorAvail = state->availMem;
        srcTape = state->memtuples[0].tupindex;
        WRITETUP(state, destTape, &state->memtuples[0]);
        /* writetup adjusted total free space, now fix per-tape space */
        spaceFreed = state->availMem - priorAvail;
        state->mergeavailmem[srcTape] += spaceFreed;
        /* compact the heap */
        tuplesort_heap_siftup(state, false);
        if ((tupIndex = state->mergenext[srcTape]) == 0)
        {
            /* out of preloaded data on this tape, try to read more */
            mergepreread(state);
            /* if still no data, we've reached end of run on this tape */
            if ((tupIndex = state->mergenext[srcTape]) == 0)
                continue;
        }
        /* pull next preread tuple from list, insert in heap */
        tup = &state->memtuples[tupIndex];
        state->mergenext[srcTape] = tup->tupindex;
        if (state->mergenext[srcTape] == 0)
            state->mergelast[srcTape] = 0;
        tuplesort_heap_insert(state, tup, srcTape, false);
        /* put the now-unused memtuples entry on the freelist */
        tup->tupindex = state->mergefreelist;
        state->mergefreelist = tupIndex;
        state->mergeavailslots[srcTape]++;
    }

    /*
     * When the heap empties, we're done.  Write an end-of-run marker on the
     * output tape, and increment its count of real runs.
     */
    markrunend(state, destTape);
    state->tp_runs[state->tapeRange]++;

#ifdef TRACE_SORT
    if (trace_sort)
        elog(LOG, "finished %d-way merge step: %s", state->activeTapes,
             pg_rusage_show(&state->ru_start));
#endif
}

/*
 * beginmerge - initialize for a merge pass
 *
 * We decrease the counts of real and dummy runs for each tape, and mark
 * which tapes contain active input runs in mergeactive[].  Then, load
 * as many tuples as we can from each active input tape, and finally
 * fill the merge heap with the first tuple from each active tape.
 */
static void
beginmerge(Tuplesortstate *state)
{
    int         activeTapes;
    int         tapenum;
    int         srcTape;
    int         slotsPerTape;
    long        spacePerTape;

    /* Heap should be empty here */
    Assert(state->memtupcount == 0);

    /* Adjust run counts and mark the active tapes */
    memset(state->mergeactive, 0,
           state->maxTapes * sizeof(*state->mergeactive));
    activeTapes = 0;
    for (tapenum = 0; tapenum < state->tapeRange; tapenum++)
    {
        if (state->tp_dummy[tapenum] > 0)
            state->tp_dummy[tapenum]--;
        else
        {
            Assert(state->tp_runs[tapenum] > 0);
            state->tp_runs[tapenum]--;
            srcTape = state->tp_tapenum[tapenum];
            state->mergeactive[srcTape] = true;
            activeTapes++;
        }
    }
    state->activeTapes = activeTapes;

    /* Clear merge-pass state variables */
    memset(state->mergenext, 0,
           state->maxTapes * sizeof(*state->mergenext));
    memset(state->mergelast, 0,
           state->maxTapes * sizeof(*state->mergelast));
    state->mergefreelist = 0;   /* nothing in the freelist */
    state->mergefirstfree = activeTapes;        /* 1st slot avail for preread */

    /*
     * Initialize space allocation to let each active input tape have an equal
     * share of preread space.
     */
    Assert(activeTapes > 0);
    slotsPerTape = (state->memtupsize - state->mergefirstfree) / activeTapes;
    Assert(slotsPerTape > 0);
    spacePerTape = state->availMem / activeTapes;
    for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
    {
        if (state->mergeactive[srcTape])
        {
            state->mergeavailslots[srcTape] = slotsPerTape;
            state->mergeavailmem[srcTape] = spacePerTape;
        }
    }

    /*
     * Preread as many tuples as possible (and at least one) from each active
     * tape
     */
    mergepreread(state);

    /* Load the merge heap with the first tuple from each input tape */
    for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
    {
        int         tupIndex = state->mergenext[srcTape];
        SortTuple  *tup;

        if (tupIndex)
        {
            tup = &state->memtuples[tupIndex];
            state->mergenext[srcTape] = tup->tupindex;
            if (state->mergenext[srcTape] == 0)
                state->mergelast[srcTape] = 0;
            tuplesort_heap_insert(state, tup, srcTape, false);
            /* put the now-unused memtuples entry on the freelist */
            tup->tupindex = state->mergefreelist;
            state->mergefreelist = tupIndex;
            state->mergeavailslots[srcTape]++;
        }
    }
}

/*
 * mergepreread - load tuples from merge input tapes
 *
 * This routine exists to improve sequentiality of reads during a merge pass,
 * as explained in the header comments of this file.  Load tuples from each
 * active source tape until the tape's run is exhausted or it has used up
 * its fair share of available memory.  In any case, we guarantee that there
 * is at least one preread tuple available from each unexhausted input tape.
 *
 * We invoke this routine at the start of a merge pass for initial load,
 * and then whenever any tape's preread data runs out.  Note that we load
 * as much data as possible from all tapes, not just the one that ran out.
 * This is because logtape.c works best with a usage pattern that alternates
 * between reading a lot of data and writing a lot of data, so whenever we
 * are forced to read, we should fill working memory completely.
 *
 * In FINALMERGE state, we *don't* use this routine, but instead just preread
 * from the single tape that ran dry.  There's no read/write alternation in
 * that state and so no point in scanning through all the tapes to fix one.
 * (Moreover, there may be quite a lot of inactive tapes in that state, since
 * we might have had many fewer runs than tapes.  In a regular tape-to-tape
 * merge we can expect most of the tapes to be active.)
 */
static void
mergepreread(Tuplesortstate *state)
{
    int         srcTape;

    for (srcTape = 0; srcTape < state->maxTapes; srcTape++)
        mergeprereadone(state, srcTape);
}

/*
 * mergeprereadone - load tuples from one merge input tape
 *
 * Read tuples from the specified tape until it has used up its free memory
 * or array slots; but ensure that we have at least one tuple, if any are
 * to be had.
 */
static void
mergeprereadone(Tuplesortstate *state, int srcTape)
{
    unsigned int tuplen;
    SortTuple   stup;
    int         tupIndex;
    long        priorAvail,
                spaceUsed;

    if (!state->mergeactive[srcTape])
        return;                 /* tape's run is already exhausted */
    priorAvail = state->availMem;
    state->availMem = state->mergeavailmem[srcTape];
    while ((state->mergeavailslots[srcTape] > 0 && !LACKMEM(state)) ||
           state->mergenext[srcTape] == 0)
    {
        /* read next tuple, if any */
        if ((tuplen = getlen(state, srcTape, true)) == 0)
        {
            state->mergeactive[srcTape] = false;
            break;
        }
        READTUP(state, &stup, srcTape, tuplen);
        /* find a free slot in memtuples[] for it */
        tupIndex = state->mergefreelist;
        if (tupIndex)
            state->mergefreelist = state->memtuples[tupIndex].tupindex;
        else
        {
            tupIndex = state->mergefirstfree++;
            Assert(tupIndex < state->memtupsize);
        }
        state->mergeavailslots[srcTape]--;
        /* store tuple, append to list for its tape */
        stup.tupindex = 0;
        state->memtuples[tupIndex] = stup;
        if (state->mergelast[srcTape])
            state->memtuples[state->mergelast[srcTape]].tupindex = tupIndex;
        else
            state->mergenext[srcTape] = tupIndex;
        state->mergelast[srcTape] = tupIndex;
    }
    /* update per-tape and global availmem counts */
    spaceUsed = state->mergeavailmem[srcTape] - state->availMem;
    state->mergeavailmem[srcTape] = state->availMem;
    state->availMem = priorAvail - spaceUsed;
}

/*
 * dumptuples - remove tuples from heap and write to tape
 *
 * This is used during initial-run building, but not during merging.
 *
 * When alltuples = false, dump only enough tuples to get under the
 * availMem limit (and leave at least one tuple in the heap in any case,
 * since puttuple assumes it always has a tuple to compare to).  We also
 * insist there be at least one free slot in the memtuples[] array.
 *
 * When alltuples = true, dump everything currently in memory.
 * (This case is only used at end of input data.)
 *
 * If we empty the heap, close out the current run and return (this should
 * only happen at end of input data).  If we see that the tuple run number
 * at the top of the heap has changed, start a new run.
 */
static void
dumptuples(Tuplesortstate *state, bool alltuples)
{
    while (alltuples ||
           (LACKMEM(state) && state->memtupcount > 1) ||
           state->memtupcount >= state->memtupsize)
    {
        /*
         * Dump the heap's frontmost entry, and sift up to remove it from the
         * heap.
         */
        Assert(state->memtupcount > 0);
        WRITETUP(state, state->tp_tapenum[state->destTape],
                 &state->memtuples[0]);
        tuplesort_heap_siftup(state, true);

        /*
         * If the heap is empty *or* top run number has changed, we've
         * finished the current run.
         */
        if (state->memtupcount == 0 ||
            state->currentRun != state->memtuples[0].tupindex)
        {
            markrunend(state, state->tp_tapenum[state->destTape]);
            state->currentRun++;
            state->tp_runs[state->destTape]++;
            state->tp_dummy[state->destTape]--; /* per Alg D step D2 */

#ifdef TRACE_SORT
            if (trace_sort)
                elog(LOG, "finished writing%s run %d to tape %d: %s",
                     (state->memtupcount == 0) ? " final" : "",
                     state->currentRun, state->destTape,
                     pg_rusage_show(&state->ru_start));
#endif

            /*
             * Done if heap is empty, else prepare for new run.
             */
            if (state->memtupcount == 0)
                break;
            Assert(state->currentRun == state->memtuples[0].tupindex);
            selectnewtape(state);
        }
    }
}

/*
 * tuplesort_rescan     - rewind and replay the scan
 */
void
tuplesort_rescan(Tuplesortstate *state)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

    Assert(state->randomAccess);

    switch (state->status)
    {
        case TSS_SORTEDINMEM:
            state->current = 0;
            state->eof_reached = false;
            state->markpos_offset = 0;
            state->markpos_eof = false;
            break;
        case TSS_SORTEDONTAPE:
            LogicalTapeRewind(state->tapeset,
                              state->result_tape,
                              false);
            state->eof_reached = false;
            state->markpos_block = 0L;
            state->markpos_offset = 0;
            state->markpos_eof = false;
            break;
        default:
            elog(ERROR, "invalid tuplesort state");
            break;
    }

    MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_markpos    - saves current position in the merged sort file
 */
void
tuplesort_markpos(Tuplesortstate *state)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

    Assert(state->randomAccess);

    switch (state->status)
    {
        case TSS_SORTEDINMEM:
            state->markpos_offset = state->current;
            state->markpos_eof = state->eof_reached;
            break;
        case TSS_SORTEDONTAPE:
            LogicalTapeTell(state->tapeset,
                            state->result_tape,
                            &state->markpos_block,
                            &state->markpos_offset);
            state->markpos_eof = state->eof_reached;
            break;
        default:
            elog(ERROR, "invalid tuplesort state");
            break;
    }

    MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_restorepos - restores current position in merged sort file to
 *                        last saved position
 */
void
tuplesort_restorepos(Tuplesortstate *state)
{
    MemoryContext oldcontext = MemoryContextSwitchTo(state->sortcontext);

    Assert(state->randomAccess);

    switch (state->status)
    {
        case TSS_SORTEDINMEM:
            state->current = state->markpos_offset;
            state->eof_reached = state->markpos_eof;
            break;
        case TSS_SORTEDONTAPE:
            if (!LogicalTapeSeek(state->tapeset,
                                 state->result_tape,
                                 state->markpos_block,
                                 state->markpos_offset))
                elog(ERROR, "tuplesort_restorepos failed");
            state->eof_reached = state->markpos_eof;
            break;
        default:
            elog(ERROR, "invalid tuplesort state");
            break;
    }

    MemoryContextSwitchTo(oldcontext);
}

/*
 * tuplesort_get_stats - extract summary statistics
 *
 * This can be called after tuplesort_performsort() finishes to obtain
 * printable summary information about how the sort was performed.
 * spaceUsed is measured in kilobytes.
 */
void
tuplesort_get_stats(Tuplesortstate *state,
                    const char **sortMethod,
                    const char **spaceType,
                    long *spaceUsed)
{
    /*
     * Note: it might seem we should provide both memory and disk usage for a
     * disk-based sort.  However, the current code doesn't track memory space
     * accurately once we have begun to return tuples to the caller (since we
     * don't account for pfree's the caller is expected to do), so we cannot
     * rely on availMem in a disk sort.  This does not seem worth the overhead
     * to fix.  Is it worth creating an API for the memory context code to
     * tell us how much is actually used in sortcontext?
     */
    if (state->tapeset)
    {
        *spaceType = "Disk";
        *spaceUsed = LogicalTapeSetBlocks(state->tapeset) * (BLCKSZ / 1024);
    }
    else
    {
        *spaceType = "Memory";
        *spaceUsed = (state->allowedMem - state->availMem + 1023) / 1024;
    }

    switch (state->status)
    {
        case TSS_SORTEDINMEM:
            if (state->boundUsed)
                *sortMethod = "top-N heapsort";
            else
                *sortMethod = "quicksort";
            break;
        case TSS_SORTEDONTAPE:
            *sortMethod = "external sort";
            break;
        case TSS_FINALMERGE:
            *sortMethod = "external merge";
            break;
        default:
            *sortMethod = "still in progress";
            break;
    }
}


/*
 * Heap manipulation routines, per Knuth's Algorithm 5.2.3H.
 *
 * Compare two SortTuples.  If checkIndex is true, use the tuple index
 * as the front of the sort key; otherwise, no.
 */

#define HEAPCOMPARE(tup1,tup2) \
    (checkIndex && ((tup1)->tupindex != (tup2)->tupindex) ? \
     ((tup1)->tupindex) - ((tup2)->tupindex) : \
     COMPARETUP(state, tup1, tup2))

/*
 * Convert the existing unordered array of SortTuples to a bounded heap,
 * discarding all but the smallest "state->bound" tuples.
 *
 * When working with a bounded heap, we want to keep the largest entry
 * at the root (array entry zero), instead of the smallest as in the normal
 * sort case.  This allows us to discard the largest entry cheaply.
 * Therefore, we temporarily reverse the sort direction.
 *
 * We assume that all entries in a bounded heap will always have tupindex
 * zero; it therefore doesn't matter that HEAPCOMPARE() doesn't reverse
 * the direction of comparison for tupindexes.
 */
static void
make_bounded_heap(Tuplesortstate *state)
{
    int         tupcount = state->memtupcount;
    int         i;

    Assert(state->status == TSS_INITIAL);
    Assert(state->bounded);
    Assert(tupcount >= state->bound);

    /* Reverse sort direction so largest entry will be at root */
    REVERSEDIRECTION(state);

    state->memtupcount = 0;     /* make the heap empty */
    for (i = 0; i < tupcount; i++)
    {
        if (state->memtupcount >= state->bound &&
          COMPARETUP(state, &state->memtuples[i], &state->memtuples[0]) <= 0)
        {
            /* New tuple would just get thrown out, so skip it */
            free_sort_tuple(state, &state->memtuples[i]);
            CHECK_FOR_INTERRUPTS();
        }
        else
        {
            /* Insert next tuple into heap */
            /* Must copy source tuple to avoid possible overwrite */
            SortTuple   stup = state->memtuples[i];

            tuplesort_heap_insert(state, &stup, 0, false);

            /* If heap too full, discard largest entry */
            if (state->memtupcount > state->bound)
            {
                free_sort_tuple(state, &state->memtuples[0]);
                tuplesort_heap_siftup(state, false);
            }
        }
    }

    Assert(state->memtupcount == state->bound);
    state->status = TSS_BOUNDED;
}

/*
 * Convert the bounded heap to a properly-sorted array
 */
static void
sort_bounded_heap(Tuplesortstate *state)
{
    int         tupcount = state->memtupcount;

    Assert(state->status == TSS_BOUNDED);
    Assert(state->bounded);
    Assert(tupcount == state->bound);

    /*
     * We can unheapify in place because each sift-up will remove the largest
     * entry, which we can promptly store in the newly freed slot at the end.
     * Once we're down to a single-entry heap, we're done.
     */
    while (state->memtupcount > 1)
    {
        SortTuple   stup = state->memtuples[0];

        /* this sifts-up the next-largest entry and decreases memtupcount */
        tuplesort_heap_siftup(state, false);
        state->memtuples[state->memtupcount] = stup;
    }
    state->memtupcount = tupcount;

    /*
     * Reverse sort direction back to the original state.  This is not
     * actually necessary but seems like a good idea for tidiness.
     */
    REVERSEDIRECTION(state);

    state->status = TSS_SORTEDINMEM;
    state->boundUsed = true;
}

/*
 * Insert a new tuple into an empty or existing heap, maintaining the
 * heap invariant.  Caller is responsible for ensuring there's room.
 *
 * Note: we assume *tuple is a temporary variable that can be scribbled on.
 * For some callers, tuple actually points to a memtuples[] entry above the
 * end of the heap.  This is safe as long as it's not immediately adjacent
 * to the end of the heap (ie, in the [memtupcount] array entry) --- if it
 * is, it might get overwritten before being moved into the heap!
 */
static void
tuplesort_heap_insert(Tuplesortstate *state, SortTuple *tuple,
                      int tupleindex, bool checkIndex)
{
    SortTuple  *memtuples;
    int         j;

    /*
     * Save the tupleindex --- see notes above about writing on *tuple. It's a
     * historical artifact that tupleindex is passed as a separate argument
     * and not in *tuple, but it's notationally convenient so let's leave it
     * that way.
     */
    tuple->tupindex = tupleindex;

    memtuples = state->memtuples;
    Assert(state->memtupcount < state->memtupsize);

    CHECK_FOR_INTERRUPTS();

    /*
     * Sift-up the new entry, per Knuth 5.2.3 exercise 16. Note that Knuth is
     * using 1-based array indexes, not 0-based.
     */
    j = state->memtupcount++;
    while (j > 0)
    {
        int         i = (j - 1) >> 1;

        if (HEAPCOMPARE(tuple, &memtuples[i]) >= 0)
            break;
        memtuples[j] = memtuples[i];
        j = i;
    }
    memtuples[j] = *tuple;
}

/*
 * The tuple at state->memtuples[0] has been removed from the heap.
 * Decrement memtupcount, and sift up to maintain the heap invariant.
 */
static void
tuplesort_heap_siftup(Tuplesortstate *state, bool checkIndex)
{
    SortTuple  *memtuples = state->memtuples;
    SortTuple  *tuple;
    int         i,
                n;

    if (--state->memtupcount <= 0)
        return;

    CHECK_FOR_INTERRUPTS();

    n = state->memtupcount;
    tuple = &memtuples[n];      /* tuple that must be reinserted */
    i = 0;                      /* i is where the "hole" is */
    for (;;)
    {
        int         j = 2 * i + 1;

        if (j >= n)
            break;
        if (j + 1 < n &&
            HEAPCOMPARE(&memtuples[j], &memtuples[j + 1]) > 0)
            j++;
        if (HEAPCOMPARE(tuple, &memtuples[j]) <= 0)
            break;
        memtuples[i] = memtuples[j];
        i = j;
    }
    memtuples[i] = *tuple;
}


/*
 * Tape interface routines
 */

static unsigned int
getlen(Tuplesortstate *state, int tapenum, bool eofOK)
{
    unsigned int len;

    if (LogicalTapeRead(state->tapeset, tapenum,
                        &len, sizeof(len)) != sizeof(len))
        elog(ERROR, "unexpected end of tape");
    if (len == 0 && !eofOK)
        elog(ERROR, "unexpected end of data");
    return len;
}

static void
markrunend(Tuplesortstate *state, int tapenum)
{
    unsigned int len = 0;

    LogicalTapeWrite(state->tapeset, tapenum, (void *) &len, sizeof(len));
}


/*
 * Inline-able copy of FunctionCall2Coll() to save some cycles in sorting.
 */
static inline Datum
myFunctionCall2Coll(FmgrInfo *flinfo, Oid collation, Datum arg1, Datum arg2)
{
    FunctionCallInfoData fcinfo;
    Datum       result;

    InitFunctionCallInfoData(fcinfo, flinfo, 2, collation, NULL, NULL);

    fcinfo.arg[0] = arg1;
    fcinfo.arg[1] = arg2;
    fcinfo.argnull[0] = false;
    fcinfo.argnull[1] = false;

    result = FunctionCallInvoke(&fcinfo);

    /* Check for null result, since caller is clearly not expecting one */
    if (fcinfo.isnull)
        elog(ERROR, "function %u returned NULL", fcinfo.flinfo->fn_oid);

    return result;
}

/*
 * Apply a sort function (by now converted to fmgr lookup form)
 * and return a 3-way comparison result.  This takes care of handling
 * reverse-sort and NULLs-ordering properly.  We assume that DESC and
 * NULLS_FIRST options are encoded in sk_flags the same way btree does it.
 */
static inline int32
inlineApplySortFunction(FmgrInfo *sortFunction, int sk_flags, Oid collation,
                        Datum datum1, bool isNull1,
                        Datum datum2, bool isNull2)
{
    int32       compare;

    if (isNull1)
    {
        if (isNull2)
            compare = 0;        /* NULL "=" NULL */
        else if (sk_flags & SK_BT_NULLS_FIRST)
            compare = -1;       /* NULL "<" NOT_NULL */
        else
            compare = 1;        /* NULL ">" NOT_NULL */
    }
    else if (isNull2)
    {
        if (sk_flags & SK_BT_NULLS_FIRST)
            compare = 1;        /* NOT_NULL ">" NULL */
        else
            compare = -1;       /* NOT_NULL "<" NULL */
    }
    else
    {
        compare = DatumGetInt32(myFunctionCall2Coll(sortFunction, collation,
                                                    datum1, datum2));

        if (sk_flags & SK_BT_DESC)
            compare = -compare;
    }

    return compare;
}


/*
 * Routines specialized for HeapTuple (actually MinimalTuple) case
 */

static int
comparetup_heap(const SortTuple *a, const SortTuple *b, Tuplesortstate *state)
{
    SortSupport sortKey = state->sortKeys;
    HeapTupleData ltup;
    HeapTupleData rtup;
    TupleDesc   tupDesc;
    int         nkey;
    int32       compare;

    /* Compare the leading sort key */
    compare = ApplySortComparator(a->datum1, a->isnull1,
                                  b->datum1, b->isnull1,
                                  sortKey);
    if (compare != 0)
        return compare;

    /* Compare additional sort keys */
    ltup.t_len = ((MinimalTuple) a->tuple)->t_len + MINIMAL_TUPLE_OFFSET;
    ltup.t_data = (HeapTupleHeader) ((char *) a->tuple - MINIMAL_TUPLE_OFFSET);
    rtup.t_len = ((MinimalTuple) b->tuple)->t_len + MINIMAL_TUPLE_OFFSET;
    rtup.t_data = (HeapTupleHeader) ((char *) b->tuple - MINIMAL_TUPLE_OFFSET);
    tupDesc = state->tupDesc;
    sortKey++;
    for (nkey = 1; nkey < state->nKeys; nkey++, sortKey++)
    {
        AttrNumber  attno = sortKey->ssup_attno;
        Datum       datum1,
                    datum2;
        bool        isnull1,
                    isnull2;

        datum1 = heap_getattr(&ltup, attno, tupDesc, &isnull1);
        datum2 = heap_getattr(&rtup, attno, tupDesc, &isnull2);

        compare = ApplySortComparator(datum1, isnull1,
                                      datum2, isnull2,
                                      sortKey);
        if (compare != 0)
            return compare;
    }

    return 0;
}

static void
copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup)
{
    /*
     * We expect the passed "tup" to be a TupleTableSlot, and form a
     * MinimalTuple using the exported interface for that.
     */
    TupleTableSlot *slot = (TupleTableSlot *) tup;
    MinimalTuple tuple;
    HeapTupleData htup;

    /* copy the tuple into sort storage */
    tuple = ExecCopySlotMinimalTuple(slot);
    stup->tuple = (void *) tuple;
    USEMEM(state, GetMemoryChunkSpace(tuple));
    /* set up first-column key value */
    htup.t_len = tuple->t_len + MINIMAL_TUPLE_OFFSET;
    htup.t_data = (HeapTupleHeader) ((char *) tuple - MINIMAL_TUPLE_OFFSET);
    stup->datum1 = heap_getattr(&htup,
                                state->sortKeys[0].ssup_attno,
                                state->tupDesc,
                                &stup->isnull1);
}

static void
writetup_heap(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
    MinimalTuple tuple = (MinimalTuple) stup->tuple;

    /* the part of the MinimalTuple we'll write: */
    char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;
    unsigned int tupbodylen = tuple->t_len - MINIMAL_TUPLE_DATA_OFFSET;

    /* total on-disk footprint: */
    unsigned int tuplen = tupbodylen + sizeof(int);

    LogicalTapeWrite(state->tapeset, tapenum,
                     (void *) &tuplen, sizeof(tuplen));
    LogicalTapeWrite(state->tapeset, tapenum,
                     (void *) tupbody, tupbodylen);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeWrite(state->tapeset, tapenum,
                         (void *) &tuplen, sizeof(tuplen));

    FREEMEM(state, GetMemoryChunkSpace(tuple));
    heap_free_minimal_tuple(tuple);
}

static void
readtup_heap(Tuplesortstate *state, SortTuple *stup,
             int tapenum, unsigned int len)
{
    unsigned int tupbodylen = len - sizeof(int);
    unsigned int tuplen = tupbodylen + MINIMAL_TUPLE_DATA_OFFSET;
    MinimalTuple tuple = (MinimalTuple) palloc(tuplen);
    char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;
    HeapTupleData htup;

    USEMEM(state, GetMemoryChunkSpace(tuple));
    /* read in the tuple proper */
    tuple->t_len = tuplen;
    LogicalTapeReadExact(state->tapeset, tapenum,
                         tupbody, tupbodylen);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeReadExact(state->tapeset, tapenum,
                             &tuplen, sizeof(tuplen));
    stup->tuple = (void *) tuple;
    /* set up first-column key value */
    htup.t_len = tuple->t_len + MINIMAL_TUPLE_OFFSET;
    htup.t_data = (HeapTupleHeader) ((char *) tuple - MINIMAL_TUPLE_OFFSET);
    stup->datum1 = heap_getattr(&htup,
                                state->sortKeys[0].ssup_attno,
                                state->tupDesc,
                                &stup->isnull1);
}

static void
reversedirection_heap(Tuplesortstate *state)
{
    SortSupport sortKey = state->sortKeys;
    int         nkey;

    for (nkey = 0; nkey < state->nKeys; nkey++, sortKey++)
    {
        sortKey->ssup_reverse = !sortKey->ssup_reverse;
        sortKey->ssup_nulls_first = !sortKey->ssup_nulls_first;
    }
}


/*
 * Routines specialized for the CLUSTER case (HeapTuple data, with
 * comparisons per a btree index definition)
 */

static int
comparetup_cluster(const SortTuple *a, const SortTuple *b,
                   Tuplesortstate *state)
{
    ScanKey     scanKey = state->indexScanKey;
    HeapTuple   ltup;
    HeapTuple   rtup;
    TupleDesc   tupDesc;
    int         nkey;
    int32       compare;

    /* Compare the leading sort key, if it's simple */
    if (state->indexInfo->ii_KeyAttrNumbers[0] != 0)
    {
        compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                          scanKey->sk_collation,
                                          a->datum1, a->isnull1,
                                          b->datum1, b->isnull1);
        if (compare != 0 || state->nKeys == 1)
            return compare;
        /* Compare additional columns the hard way */
        scanKey++;
        nkey = 1;
    }
    else
    {
        /* Must compare all keys the hard way */
        nkey = 0;
    }

    /* Compare additional sort keys */
    ltup = (HeapTuple) a->tuple;
    rtup = (HeapTuple) b->tuple;

    if (state->indexInfo->ii_Expressions == NULL)
    {
        /* If not expression index, just compare the proper heap attrs */
        tupDesc = state->tupDesc;

        for (; nkey < state->nKeys; nkey++, scanKey++)
        {
            AttrNumber  attno = state->indexInfo->ii_KeyAttrNumbers[nkey];
            Datum       datum1,
                        datum2;
            bool        isnull1,
                        isnull2;

            datum1 = heap_getattr(ltup, attno, tupDesc, &isnull1);
            datum2 = heap_getattr(rtup, attno, tupDesc, &isnull2);

            compare = inlineApplySortFunction(&scanKey->sk_func,
                                              scanKey->sk_flags,
                                              scanKey->sk_collation,
                                              datum1, isnull1,
                                              datum2, isnull2);
            if (compare != 0)
                return compare;
        }
    }
    else
    {
        /*
         * In the expression index case, compute the whole index tuple and
         * then compare values.  It would perhaps be faster to compute only as
         * many columns as we need to compare, but that would require
         * duplicating all the logic in FormIndexDatum.
         */
        Datum       l_index_values[INDEX_MAX_KEYS];
        bool        l_index_isnull[INDEX_MAX_KEYS];
        Datum       r_index_values[INDEX_MAX_KEYS];
        bool        r_index_isnull[INDEX_MAX_KEYS];
        TupleTableSlot *ecxt_scantuple;

        /* Reset context each time to prevent memory leakage */
        ResetPerTupleExprContext(state->estate);

        ecxt_scantuple = GetPerTupleExprContext(state->estate)->ecxt_scantuple;

        ExecStoreTuple(ltup, ecxt_scantuple, InvalidBuffer, false);
        FormIndexDatum(state->indexInfo, ecxt_scantuple, state->estate,
                       l_index_values, l_index_isnull);

        ExecStoreTuple(rtup, ecxt_scantuple, InvalidBuffer, false);
        FormIndexDatum(state->indexInfo, ecxt_scantuple, state->estate,
                       r_index_values, r_index_isnull);

        for (; nkey < state->nKeys; nkey++, scanKey++)
        {
            compare = inlineApplySortFunction(&scanKey->sk_func,
                                              scanKey->sk_flags,
                                              scanKey->sk_collation,
                                              l_index_values[nkey],
                                              l_index_isnull[nkey],
                                              r_index_values[nkey],
                                              r_index_isnull[nkey]);
            if (compare != 0)
                return compare;
        }
    }

    return 0;
}

static void
copytup_cluster(Tuplesortstate *state, SortTuple *stup, void *tup)
{
    HeapTuple   tuple = (HeapTuple) tup;

    /* copy the tuple into sort storage */
    tuple = heap_copytuple(tuple);
    stup->tuple = (void *) tuple;
    USEMEM(state, GetMemoryChunkSpace(tuple));
    /* set up first-column key value, if it's a simple column */
    if (state->indexInfo->ii_KeyAttrNumbers[0] != 0)
        stup->datum1 = heap_getattr(tuple,
                                    state->indexInfo->ii_KeyAttrNumbers[0],
                                    state->tupDesc,
                                    &stup->isnull1);
}

static void
writetup_cluster(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
    HeapTuple   tuple = (HeapTuple) stup->tuple;
    unsigned int tuplen = tuple->t_len + sizeof(ItemPointerData) + sizeof(int);

    /* We need to store t_self, but not other fields of HeapTupleData */
    LogicalTapeWrite(state->tapeset, tapenum,
                     &tuplen, sizeof(tuplen));
    LogicalTapeWrite(state->tapeset, tapenum,
                     &tuple->t_self, sizeof(ItemPointerData));
    LogicalTapeWrite(state->tapeset, tapenum,
                     tuple->t_data, tuple->t_len);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeWrite(state->tapeset, tapenum,
                         &tuplen, sizeof(tuplen));

    FREEMEM(state, GetMemoryChunkSpace(tuple));
    heap_freetuple(tuple);
}

static void
readtup_cluster(Tuplesortstate *state, SortTuple *stup,
                int tapenum, unsigned int tuplen)
{
    unsigned int t_len = tuplen - sizeof(ItemPointerData) - sizeof(int);
    HeapTuple   tuple = (HeapTuple) palloc(t_len + HEAPTUPLESIZE);

    USEMEM(state, GetMemoryChunkSpace(tuple));
    /* Reconstruct the HeapTupleData header */
    tuple->t_data = (HeapTupleHeader) ((char *) tuple + HEAPTUPLESIZE);
    tuple->t_len = t_len;
    LogicalTapeReadExact(state->tapeset, tapenum,
                         &tuple->t_self, sizeof(ItemPointerData));
    /* We don't currently bother to reconstruct t_tableOid */
    tuple->t_tableOid = InvalidOid;
    /* Read in the tuple body */
    LogicalTapeReadExact(state->tapeset, tapenum,
                         tuple->t_data, tuple->t_len);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeReadExact(state->tapeset, tapenum,
                             &tuplen, sizeof(tuplen));
    stup->tuple = (void *) tuple;
    /* set up first-column key value, if it's a simple column */
    if (state->indexInfo->ii_KeyAttrNumbers[0] != 0)
        stup->datum1 = heap_getattr(tuple,
                                    state->indexInfo->ii_KeyAttrNumbers[0],
                                    state->tupDesc,
                                    &stup->isnull1);
}


/*
 * Routines specialized for IndexTuple case
 *
 * The btree and hash cases require separate comparison functions, but the
 * IndexTuple representation is the same so the copy/write/read support
 * functions can be shared.
 */

static int
comparetup_index_btree(const SortTuple *a, const SortTuple *b,
                       Tuplesortstate *state)
{
    /*
     * This is similar to _bt_tuplecompare(), but we have already done the
     * index_getattr calls for the first column, and we need to keep track of
     * whether any null fields are present.  Also see the special treatment
     * for equal keys at the end.
     */
    ScanKey     scanKey = state->indexScanKey;
    IndexTuple  tuple1;
    IndexTuple  tuple2;
    int         keysz;
    TupleDesc   tupDes;
    bool        equal_hasnull = false;
    int         nkey;
    int32       compare;

    /* Compare the leading sort key */
    compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                      scanKey->sk_collation,
                                      a->datum1, a->isnull1,
                                      b->datum1, b->isnull1);
    if (compare != 0)
        return compare;

    /* they are equal, so we only need to examine one null flag */
    if (a->isnull1)
        equal_hasnull = true;

    /* Compare additional sort keys */
    tuple1 = (IndexTuple) a->tuple;
    tuple2 = (IndexTuple) b->tuple;
    keysz = state->nKeys;
    tupDes = RelationGetDescr(state->indexRel);
    scanKey++;
    for (nkey = 2; nkey <= keysz; nkey++, scanKey++)
    {
        Datum       datum1,
                    datum2;
        bool        isnull1,
                    isnull2;

        datum1 = index_getattr(tuple1, nkey, tupDes, &isnull1);
        datum2 = index_getattr(tuple2, nkey, tupDes, &isnull2);

        compare = inlineApplySortFunction(&scanKey->sk_func, scanKey->sk_flags,
                                          scanKey->sk_collation,
                                          datum1, isnull1,
                                          datum2, isnull2);
        if (compare != 0)
            return compare;     /* done when we find unequal attributes */

        /* they are equal, so we only need to examine one null flag */
        if (isnull1)
            equal_hasnull = true;
    }

    /*
     * If btree has asked us to enforce uniqueness, complain if two equal
     * tuples are detected (unless there was at least one NULL field).
     *
     * It is sufficient to make the test here, because if two tuples are equal
     * they *must* get compared at some stage of the sort --- otherwise the
     * sort algorithm wouldn't have checked whether one must appear before the
     * other.
     */
    if (state->enforceUnique && !equal_hasnull)
    {
        Datum       values[INDEX_MAX_KEYS];
        bool        isnull[INDEX_MAX_KEYS];

        /*
         * Some rather brain-dead implementations of qsort (such as the one in
         * QNX 4) will sometimes call the comparison routine to compare a
         * value to itself, but we always use our own implementation, which
         * does not.
         */
        Assert(tuple1 != tuple2);

        index_deform_tuple(tuple1, tupDes, values, isnull);
        ereport(ERROR,
                (errcode(ERRCODE_UNIQUE_VIOLATION),
                 errmsg("could not create unique index \"%s\"",
                        RelationGetRelationName(state->indexRel)),
                 errdetail("Key %s is duplicated.",
                           BuildIndexValueDescription(state->indexRel,
                                                      values, isnull)),
                 errtableconstraint(state->heapRel,
                                 RelationGetRelationName(state->indexRel))));
    }

    /*
     * If key values are equal, we sort on ItemPointer.  This does not affect
     * validity of the finished index, but it may be useful to have index
     * scans in physical order.
     */
    {
        BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
        BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);

        if (blk1 != blk2)
            return (blk1 < blk2) ? -1 : 1;
    }
    {
        OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
        OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);

        if (pos1 != pos2)
            return (pos1 < pos2) ? -1 : 1;
    }

    return 0;
}

static int
comparetup_index_hash(const SortTuple *a, const SortTuple *b,
                      Tuplesortstate *state)
{
    uint32      hash1;
    uint32      hash2;
    IndexTuple  tuple1;
    IndexTuple  tuple2;

    /*
     * Fetch hash keys and mask off bits we don't want to sort by. We know
     * that the first column of the index tuple is the hash key.
     */
    Assert(!a->isnull1);
    hash1 = DatumGetUInt32(a->datum1) & state->hash_mask;
    Assert(!b->isnull1);
    hash2 = DatumGetUInt32(b->datum1) & state->hash_mask;

    if (hash1 > hash2)
        return 1;
    else if (hash1 < hash2)
        return -1;

    /*
     * If hash values are equal, we sort on ItemPointer.  This does not affect
     * validity of the finished index, but it may be useful to have index
     * scans in physical order.
     */
    tuple1 = (IndexTuple) a->tuple;
    tuple2 = (IndexTuple) b->tuple;

    {
        BlockNumber blk1 = ItemPointerGetBlockNumber(&tuple1->t_tid);
        BlockNumber blk2 = ItemPointerGetBlockNumber(&tuple2->t_tid);

        if (blk1 != blk2)
            return (blk1 < blk2) ? -1 : 1;
    }
    {
        OffsetNumber pos1 = ItemPointerGetOffsetNumber(&tuple1->t_tid);
        OffsetNumber pos2 = ItemPointerGetOffsetNumber(&tuple2->t_tid);

        if (pos1 != pos2)
            return (pos1 < pos2) ? -1 : 1;
    }

    return 0;
}

static void
copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup)
{
    IndexTuple  tuple = (IndexTuple) tup;
    unsigned int tuplen = IndexTupleSize(tuple);
    IndexTuple  newtuple;

    /* copy the tuple into sort storage */
    newtuple = (IndexTuple) palloc(tuplen);
    memcpy(newtuple, tuple, tuplen);
    USEMEM(state, GetMemoryChunkSpace(newtuple));
    stup->tuple = (void *) newtuple;
    /* set up first-column key value */
    stup->datum1 = index_getattr(newtuple,
                                 1,
                                 RelationGetDescr(state->indexRel),
                                 &stup->isnull1);
}

static void
writetup_index(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
    IndexTuple  tuple = (IndexTuple) stup->tuple;
    unsigned int tuplen;

    tuplen = IndexTupleSize(tuple) + sizeof(tuplen);
    LogicalTapeWrite(state->tapeset, tapenum,
                     (void *) &tuplen, sizeof(tuplen));
    LogicalTapeWrite(state->tapeset, tapenum,
                     (void *) tuple, IndexTupleSize(tuple));
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeWrite(state->tapeset, tapenum,
                         (void *) &tuplen, sizeof(tuplen));

    FREEMEM(state, GetMemoryChunkSpace(tuple));
    pfree(tuple);
}

static void
readtup_index(Tuplesortstate *state, SortTuple *stup,
              int tapenum, unsigned int len)
{
    unsigned int tuplen = len - sizeof(unsigned int);
    IndexTuple  tuple = (IndexTuple) palloc(tuplen);

    USEMEM(state, GetMemoryChunkSpace(tuple));
    LogicalTapeReadExact(state->tapeset, tapenum,
                         tuple, tuplen);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeReadExact(state->tapeset, tapenum,
                             &tuplen, sizeof(tuplen));
    stup->tuple = (void *) tuple;
    /* set up first-column key value */
    stup->datum1 = index_getattr(tuple,
                                 1,
                                 RelationGetDescr(state->indexRel),
                                 &stup->isnull1);
}

static void
reversedirection_index_btree(Tuplesortstate *state)
{
    ScanKey     scanKey = state->indexScanKey;
    int         nkey;

    for (nkey = 0; nkey < state->nKeys; nkey++, scanKey++)
    {
        scanKey->sk_flags ^= (SK_BT_DESC | SK_BT_NULLS_FIRST);
    }
}

static void
reversedirection_index_hash(Tuplesortstate *state)
{
    /* We don't support reversing direction in a hash index sort */
    elog(ERROR, "reversedirection_index_hash is not implemented");
}


/*
 * Routines specialized for DatumTuple case
 */

static int
comparetup_datum(const SortTuple *a, const SortTuple *b, Tuplesortstate *state)
{
    return ApplySortComparator(a->datum1, a->isnull1,
                               b->datum1, b->isnull1,
                               state->onlyKey);
}

static void
copytup_datum(Tuplesortstate *state, SortTuple *stup, void *tup)
{
    /* Not currently needed */
    elog(ERROR, "copytup_datum() should not be called");
}

static void
writetup_datum(Tuplesortstate *state, int tapenum, SortTuple *stup)
{
    void       *waddr;
    unsigned int tuplen;
    unsigned int writtenlen;

    if (stup->isnull1)
    {
        waddr = NULL;
        tuplen = 0;
    }
    else if (state->datumTypeByVal)
    {
        waddr = &stup->datum1;
        tuplen = sizeof(Datum);
    }
    else
    {
        waddr = DatumGetPointer(stup->datum1);
        tuplen = datumGetSize(stup->datum1, false, state->datumTypeLen);
        Assert(tuplen != 0);
    }

    writtenlen = tuplen + sizeof(unsigned int);

    LogicalTapeWrite(state->tapeset, tapenum,
                     (void *) &writtenlen, sizeof(writtenlen));
    LogicalTapeWrite(state->tapeset, tapenum,
                     waddr, tuplen);
    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeWrite(state->tapeset, tapenum,
                         (void *) &writtenlen, sizeof(writtenlen));

    if (stup->tuple)
    {
        FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
        pfree(stup->tuple);
    }
}

static void
readtup_datum(Tuplesortstate *state, SortTuple *stup,
              int tapenum, unsigned int len)
{
    unsigned int tuplen = len - sizeof(unsigned int);

    if (tuplen == 0)
    {
        /* it's NULL */
        stup->datum1 = (Datum) 0;
        stup->isnull1 = true;
        stup->tuple = NULL;
    }
    else if (state->datumTypeByVal)
    {
        Assert(tuplen == sizeof(Datum));
        LogicalTapeReadExact(state->tapeset, tapenum,
                             &stup->datum1, tuplen);
        stup->isnull1 = false;
        stup->tuple = NULL;
    }
    else
    {
        void       *raddr = palloc(tuplen);

        LogicalTapeReadExact(state->tapeset, tapenum,
                             raddr, tuplen);
        stup->datum1 = PointerGetDatum(raddr);
        stup->isnull1 = false;
        stup->tuple = raddr;
        USEMEM(state, GetMemoryChunkSpace(raddr));
    }

    if (state->randomAccess)    /* need trailing length word? */
        LogicalTapeReadExact(state->tapeset, tapenum,
                             &tuplen, sizeof(tuplen));
}

static void
reversedirection_datum(Tuplesortstate *state)
{
    state->onlyKey->ssup_reverse = !state->onlyKey->ssup_reverse;
    state->onlyKey->ssup_nulls_first = !state->onlyKey->ssup_nulls_first;
}

/*
 * Convenience routine to free a tuple previously loaded into sort memory
 */
static void
free_sort_tuple(Tuplesortstate *state, SortTuple *stup)
{
    FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
    pfree(stup->tuple);
}